1994 GMC K1500. Ran great, blew head gasket. Still ran but with miss. Replaced stock heads with rebuilt heads, new pushrods, new distributor, new coil. Now has hard start condition, but idles fine once started. Once it gets up to op temp, idles very rough, won't take fuel, eventually dies. Won't restart til cold. Sets code referencing ignition module (according to Chiltons). Replaced module with known good module, no change - replaced distributor & coil - still no change.

As per the problem which you mentioned in your question it says " Once it gets up to op temp, idles very rough, won't take fuel, eventually dies. Won't restart til cold."------- ------- Its low fuel pressure or problem with o2 sensor.----------

As per your problem mentioned, it indicates the problem with fuel pressure.-------- The fuel pressure is low.---------- First thing you can do is get the fuel pressure checked, this will confirm the problem. --------- If the pressure checked low that means problem is with faulty pump and clogged fuel filter.----------- If the fuel pressure is checked ok, then other possibility is faulty o2 sensor and crank sensor. The o2 sensors are before and after catalytic converter.-------Both the sensors are to be replaced.------ And as per you mentioned your problem, its quite possible that faulty o2 sensor will cause this problem.------ O2 sensor is the part which sends signal to ECM( the cars computer).It sends signal to computer about fuel / air mixture.How and when to send fuel to engine.--------- If o2 sends stops sending signal to cars computer then it will not send the fuel to engine.------- In your case the o2 sensor is getting overheated and this is stopping the o2 sensor from sending signal to ECM.But when your car shuts off the sensor gets cool and again it starts working.------ So as per problem, get both the o2 sensors checked and replaced.---------- Also get the crank sensor checked.--------This will help.Thanks.Helpmech.