For about the last year, I've had a problem keeping my bike on the road. Originally, my rear tube went flat when the tire was punctured by an office staple (don't ask me how a regular office staple could have been positioned properly on the road and puncture all the way through the tire while I was riding, I can't figure that out either). Since then, every time I replace the tube, it lasts for maybe two weeks and then goes flat again. Each time, the tube gets this mysterious slice along the inner side. Here is a picture of one of these slices. It's very faint; only deep enough to puncture fully through the tube in a few places. This is a picture of the inner side of the tube. The slice is almost halfway down from the seam and runs across the length of the tube shown here. My patch is covering the largest hole, but doesn't solve the problem.
The first time I saw this, I figured it was something to do with the rim. However, this slice (which is usually about 20cm long) appears in different locations relative to the valve on every new tube. So if it is something to do with the rim, it isn't anything that would be in a fixed location on the rim.
I also thought it could be due to installation error, so I made sure a few times I had professionals at the shop do the installing. Same thing; about two weeks later it would be flat with a slice like this through it.
Obviously, this has nothing to do with the tire itself, since no part of the tire is in contact with this part of the tube. Given the time it takes to form, it looks like something is gradually cutting into the tube and, after about two weeks, finally manages to wear through it. I've washed the rim thoroughly and run my fingers along it to check for slivers. Nothing. I thought maybe it was the rim strip, the edge of which contacts the tube. That's still my number 1 guess, but if that's it, why is this slice only along a ~20cm section of the tube (I've checked the rest, no slice) and always in a different place along the rim? Why wouldn't it either be in the same place or along the entire tube if it were the strip?
Anyway, this mystery has me stumped and has cost me a fortune in tubes and bus fare (what with a bike no longer being my commute option). To try to mitigate it, I've tried using self-sealing tubes, which obviously didn't work perfectly (lasted an extra few days though) because they are more suited for punctures on the outer side of the tube. My next thought is trying puncture-proof tubes. If I can't figure out how to stop these slices, I figure puncture-proof tubes would be thick enough to give me a few extra weeks per tube.
For the record, the bike is a Trek 3700 with 26X2 wheels and I usually pump the rear wheel up to about 45-50 psi.
TL;DR: What's causing these slices in my tubes? How can I prevent it?