I'm a 44 year-old male, fast-recreational cyclist -- about 6,000 miles/yr. -- who has used a heart rate monitor on and off for nearly 15 years. Quite honestly, I find it kind of aggravating, as it often seems like my heart knows what I want to do and refuses to do it! If trying to do a "recovery ride" my HR can rise so easily that its next to impossible to stay in the recovery zone, yet, when trying to do intervals, I often struggle to get to 150 BPM. I recently got an indoor trainer which had a built in power meter. (My first time using one.) Out of curiosity, this past week I decided that everyday, I would ride at a steady 200 watts -- which seems to be upper endurance or lower tempo zone for me -- for a while to see how it correlates to my heart rate. While I'm fully aware of how HR varies for numerous reasons, even I was surprised that my HR ranged from 123 BPM to 152 BPM for that same effort! (And both those reading were on consecutive days.) I don't do anything really structured, but have recently been following the basic concepts of the CTS training routines. (Doing their "field test" -- which ranged from 148 - 164 BPM for my tests -- then setting my zones accordingly.) Now, how in the heck am I supposed to do something like CTS's "climbing repeat intervals" -- which has a range of just 4 BPM -- if my HR varies by as much as THIRTY BPM for a given effort?!?! (Yes, I realize a power meter on a trainer may not be 100% accurate, but still.) In fact, if my HR varies that much, wouldn't that mean that, most of the time, while doing a "climbing repeat," I wouldn't be at the correct intensity, even if I strictly adhered to the "correct" HR zone? (I tried getting an LT test at the CTS Lab, but that failed . . . they couldn't draw enough blood to get accurate readings.) Looking back at my training logs, it seems I may have ridden better when just riding by feel. At the least, it was more enjoyable riding that way, rather than always struggling to stay in a prescribed zone. ("Oh, slow down, my HR is too high . . . wait, now it's too low, pick up the pace ... too high . . . too low . . . too high . . . ") Would I be better off going back to that? (I still had some structure, in terms of "recovery," "endurance," and "interval" days. I just didn't check or worry about being at a specific HR.) A power meter would be nice, but unless the prices came WAY down, forget it! If I do stay with the HR zone training, should I adjust the zones on a daily basis, based on whether my HR is high or low? (It becomes quickly apparent to me if its high or low. But the only way I would know by how much is to check on the indoor trainer before the ride.) But, hey, Eddy Merckx never used a heart rate monitor. Seems like he didn't do too bad . . .
Based on your description of your issues with heart rate training, I'd be willing to bet that you are training daily, or without enough rest days between sessions, and that you are training above your cardio fitness level.
If you have a need to train to a goal, rather than just ride, try dropping the intensity of your training down a level, and make sure that you follow a good rest day schedule as well.
Our bodies don't gain during exercise. Exercise breaks us down. The gains come in the rest periods after exercise, when your body recognizes that it's been put through the ringer, and builds up to a peak that will support the new level of effort that is required.
The problem is that this peak effort stage only lasts a short period of time before your body goes back to what it's used to, and it's tricky to figure out that recovery period for yourself. The key is that if you exercise again at that recovery point, and then have a rest period, then you will make gains.
Push yourself again too early, and you prevent the build up from happening. Wait too long, and the gains are gone before you build on them.
Why is this relevant to your question? Because if you are seeing very different heart rates (especially higher rates) on the same course and level of effort over 2 consecutive days, then your body isn't finished with its building stage. You needed more rest before you push it again.
For me, I make the most gains when I ride hard, and then rest 2 days, another hard effort, then rest one day. Light effort, one day rest. Hard effort, 2 days rest. And so on.
But, and this is key, it's different for everyone. Following someone else's plan is good to figure out what works for you, but you will need to pay attention to the feedback your body gives you and modify accordingly. That feedback (at least in the cycling/training world) comes in the form of data from your heart rate and power meters, average speed, and perceived effort levels.
CTS has great training programs which are designed to be applicable to a lot of people. They may need to be modified to work well with your body.
This article may provide a bit of insight into some of the factors:
(It's running-oriented, but applicable to cyclists as well.)
Dehydration has a huge impact on my heart rate. I've seen it rise by 20 points or more when I'm badly dehydrated. Training within an hour or two of a meal also seems to raise my heart rate.
That said, the range of heart rates you're seeing seems suspicious. Have you tried using another HRM to check the accuracy of yours? FWIW, my HRM (a Garmin 500) shows erratic results if the strap contacts are insufficiently moistened or badly positioned on my chest.