Sounds like head gasket is on the way out. Called a head gasket swing I believe.

 

My S10 was doing all that too when the headgasket went out. The problem is that air gets in and sometimes the sending unit won't be touching hot water because it's not flowing properly.

But it can also be a bad thermostat, stuck mostly open so you get good flow but not 100% which would explain the slightly high temp under load and the super low temp on coasting.