engine problems after 20 minutes

[spannercutrona]

i am in a bit of a bother here and not sure whats going on with the car...i was having some problems with the ignition module where i was only running on 3 cylinders... i have fixed this problem by replacing one of the plugs on the module... i also changed all the spark plugs for platinum plugs... the engine was running fine after this... but ive just noticed after say about 15-20 minutes of driving the engine starts to run rough as in really shakey again... if anyone has got any ideas as to whats wrong please let me know what they think

thanx

[PapaSmurf2k3]

Probably a coolant temp sensor, but you might want to try doing a compression check after it starts doing it... I fear you might have a warped head or something.

[aris_unlimited]

Are you getting any engine codes?