No overdrive or low

I have a 1997 GMC Jimmy w/ 101,000 miles. About a month ago I was driving down the road and lost overdrive. The service engine soon light then came on. I stopped at Autozone and their computer said TCC switch. I started up and everything was ok. It now happens about every 150 miles. I have no low gear or overdive. I also cannot get the key out of the switch while this is happening. After I drive 4 or 5 miles and turn off ignition, I can turn the key off/ on 2 or 3 times and then all is well for another 150 miles. When the problem is not present I have no slippage of anykind & 4WD works just fine. Any ideas?

The brake switch system will trigger a TCC dropout by the look of things.

At that mileage, I'd suspect the wiring/connectors as the first to be eliminated from the loop.