I have a 1997 Gary Fisher Tassajara that got a complete overhaul and tune-up at a bike shop a year ago. (to the tune of $180, but some of that was new tires- I'm the original owner) I ride between 4 and 8 miles on logging roads and easy dirt trails several times a week, and didn't have this issue until late Spring. The slipping started infrequently, but has gotten much worse in the past two months. It doesn't slip in the highest gear, and I can sometimes find a lower one that will work for me- but other times I end up having to put the chain back on mid-ride. I'm a little worried about the chain or gears actually breaking while I'm out in the wilderness alone, so have limited my backwoods trail-taking until I get this issue taken care of.
When your bike was overhauled they likely replaced the shifter cables. The cables will stretch over time. The result is the derailleurs don't shift as accurately as they should. It is a fairly simple process to readjust them. There are many on line tutorials that will show you how to do this yourself. If you don't feel comfortable doing it yourself your local shop can help you.
When riding in wet/dirty terrain it's important to make sure your chain is well oiled and operating smoothly. If the chain isn't clean and lubricated it will feel like it "slips". If it feels stiff in a couple places after cleaning, lubrication, and a ride around the block, it might be time to replace it. A thorough cleaning and good lubrication only works if your derailleurs are aligned properly though. Check your derailleurs and make sure they aren't overshooting the correct gears. Cables stretch and often times I find myself tuning my derailleurs at least once after new cables are installed.
This is a textbook cable tension issue. Since you stated that your highest and lowest gears are working well, I assume that the middle gears are the ones that are slipping. You could take it back to the bike shop and explain this to them and they'll surely fix it for you. Given the time that has passed since they last touched it, they'll probably charge you. If you want to just fix it yourself, use the thumb and index-finger adjusters either up near your shifter or down at the derailleur. First, shift to a gear that slips, preferably the most centered gear on your cassette. Looking down onto your bike from above, or from behind the back tire, align your eye with your chainline and your cassette. Twist the cable tension adjuster to increase or decrease the cable tension and watch the derailleur move away from, or toward your spokes. Center up the line that the chain is running by watching your cassette and the derailleur pulleys. These will likely be very small adjustments! Center it up and then jump on and go through the gears. If you get it close, then use the adjuster up near your shifter to fine tune it while you pedal. If you can't get it, the cable might have stretched to the point where it needs to be reset at its anchor point at the derailleur. Bike shops should know to stretch all cables when they are installed and test ride everything, but this is a good thing to know how to do.