Overheating Problems

[Rebustillo]

I own a 1994 GMC K-1500 4WD with a 350 4-bolt main engine.It has a towing cam installed and is bored .30 over.The heads were ported and polished.I have installed a Hypertech chip with a 160 degree thermostat,headers,an aftermarket distributer module with an MSD coil and wires.It is a TBI system with a spacer for the injectors and a throttle body spacer. It also has a throttle sensor enhancer.I am running a little hotter plug in it than stock.
The problem I am having is that after the engine warms up and i am running down the road, the temperature gauge will start to climb above what the thermostat is set for.This does this at no set pattern or time or condition.It just happens out of the blue.It will climb until I pull over and kick it out of gear and sit at idle then it will return to normal operating temp.I have a new water pump,thermostat,coolant and temp.sensor.I have heard that ignition timing will sometimes afffect the temp.I have this engine set at 1 degree after TDC.It calls for it to be set at TDC.I am stumped. I would appreciate any input on this.

[Rebustillo]

Well I got it figured out.I ran the motor up to operating temp,then took the radiator cap loose while the motor was running and saw bubbles in the coolant.I had a head gasket that was leaking.So instead of tearing it down to replace it ,I retorqued the headbolts and that solved the problem.

[coachmill]

I am having the same problem on my '98. All parts are stock. I've replaced both the water pump and the thermostate. Could it be the head gasket? Would I need to replace this or just have it torqued?