I now have a few flight controllers that seem to have bad gyros, and I'm trying to figure out if there is anything I can do to repair them. One is on a Flywoo F411 16x16 FC, and the other is on an original FPVCycle Zeez 1S AIO. On the Zeez board, as soon as power is applied, the gyro shows the quad rotating around the Z (yaw) axis by a few degrees a second. On the Flywoo board, it seems to be temperature dependent as the gyro signal is initially stable then after a short while begins showing rotation around multiple axis, again at a few degrees per second. 

Both use MPU6000 gyros. I have checked this pins for anything that could be touching and tried reflowing the joints without any improvement. 

Any ideas? Do gyros just go bad after awhile?

I don't honestly know, but I guess the first question to be asked is have those FC's ever been crashed?

Both boards were on used quads I picked up cheap, so probably

I've been buying broken quads to repair as something to do, and most fixes have been pretty straightforward (dirty motors, bad soldering, wrong FW flashed, etc). It's just these two boards with gyro issues that are giving me a headache trying to troubleshoot. A quick search didn't find many mentions of gyros going bad like this, so I didn't think it was a common problem.

You could give it a try to account for it via the radio output for yaw. Adjust it until the model of the quad in betaflight stays still.

Doesn't fix the gyro if there is a problem. But could make it flyable as long as you adjust for that with every radio used on the quad.