About every 10 seconds. Its a 98 civic lx. But idles fine with the ac off and runs fine with the ac on or off. Any ideas on how to fix this?

 

Is the CEL ON?

Three items to consider:

1) Idle speed adjustment too low - check idle adjustment screw on the throttle body

2) Malfunctioning IACV on throttle body - try cleaning (see forum DIY)

3) A problem with the A/C signal to the ECU - the A/C system sends a signal to the ECU to increase idle speed. There could be a problem with this circuit.

 

check the stuff stated above ^^^,

but also something to consider is that the compressor turns itself on and off while its turned on at the a/c controls. so the engine might just be shuddering when the compressor turns back on and the engine takes a second to catch up with the extra load that the compressor puts on it.

you can notice this on most four cylinder cars, but it is only a minor fluxuation, nothing crazy. but if your car is doing it very bad, then there may be a problem somwhere in the a/c system.

 

also, have you recently put in more refrigerant? You might have put in too much