1992 Jimmy temp gauge and idle problem

ok I have a 1992 gmc jimmy with the vortec code w engine about 2 months ago my temp gauge stated to act funny it would spike up to 260 and come right back down to about 180 now as soon as you turn the car on it jumps to 260 i have replaced the temp senoir, cooling hoses, coolent fuild and added a low temp thermostate and none of it has helped i thought it could be a short in the wire somewhere but i haven't been able to find anything that looks out of place. Can some body give me some ideas. Thanks Also i burn a little oil if i let my jimmy idle it dosen't seen to burn if i'm at higher RPMS plus my idle gets pretty low it almost stalls is there a way to increase the idle speed thanks for looking again

Sounds like either an air pocket in the cooling system or a clogged heater core...

Ok what do i have to do to fix it

bleed it if it's air, flush it if it's clogged... Best way to bleed it is to warm the engine to operating temp then pull the temp gauge on the driver's side of block, fill antifereze till it's pouring out of that hole, put sensor back in, restart car, start filling the rad (make sure that heat is on during this whole process. If this doesnt work, pull the hoses off the heater core against the fire wall and flush it hard with a hose...