my 88 GTU has a miss... sometimes
 

i never noticed too much of a miss in my GTU till just before i bought my full racing beat exhaust. so now that each rotor has its own piping i can tell that its the front rotor that has the miss.
the strange thing is that when i first start my car she idles fine but then after a minute or two i can hear the idle pick up a little like something turned on, and she starts to miss out of that front rotor.
anyone have any ideas?

come on people doesnt anyone know why this is happening or have any suggestions or something?

I'm by no means a guru but have you checked for ECU error codes?

Have you changed sparkplug/sparkplug wires? If you haven't, then you might need to do this. Changing the exhaust can cause the the engine rich on S4 engines. Also, check the compression of the front rotor. ECU error codes are also a good way to start.