Truck is driving me insane.....another problem!

So I was at work yesterday in my truck with it idling, the temp outside was probably 50 - 55 degree's and I was chatting with a co-worker probably idled 10-15 min and looked over at my gauges and the temp gauge was going wayyy up so I shut the truck off immediately. Let the truck sit for about 30 minutes and then fired her back up drove around and in no time she started heating up and quick so I turned on the heater, it blew hot for a few seconds while the temp gauge went down slightly then all of the sudden it started blowing cold air and the truck started getting hot again.

That was about 2 pm ish so I let the truck sit till i got off from work at 4:30. Well it was that time and I dreaded the drive home and the stopping every few minutes to let her cool down but it was opposite she heated up a little at the very first light I hit and then the temp gauge went cold and has stayed there since.

So right now my problems are that my temp gauge is not going to running temp at all it stays cold even after freeway driving and idling in 60-70 degree weather. Also my heater still blows cold and my idle has gone up a bit in park she idles at about 900 rpms and in drive at about 700 - 800 when I let off the gas while driving she goes up to around 1k+ rpm.

Coolant is good and the thermostat is 3 months old...replaced thermostat when i did the plenum and flushed and changed coolant at the same time.

It is a 98 with the 5.2L