2005 Chevrolet Impala Engine coolant temperature sensor

I have been having many problems with my cooling system. Two years ago, my ex put in one of those 'stop leak' crayons to fix what we thought was a leak in my rad. (Turned out it was a worn hose on my water pump that was leaking). Recently, I had noticed that my temp gauge was only going halfway to the 'halfway mark. Then, some days it would go to half way, then kick in and go back down when it went a bit over the half mark. My mechanic thinks that my ECT sensors are clogged from the stop leak crayon, but that they could also be damaged from this. Can I still overheat with my temperature gauge working sporadically? I had a rad flush few days ago, but still my temp gauge is acting up. Tonight when I was driving, I noticed that the dash lights for the temp gauge were not working, so I banged on the dash and they came back on.