The fan on my 92 Legacy will intermittently cycle on and off very fast when the AC is enganged, it sounds like a bad relay or solenoid switiching on and off very fast, it will sometimes cause the car to buck a little.

 

Any ideas?

Do you hear a clicking when it happens?

Yes,

 

Click , click .....click click. The bucking seems to match the clicks

It sounds like the AC relay is clicking on and off....which is loading/unloading the engine....which may be causing the bucking.

 

Do you have any issues when the AC is not on?

 

Also, by fan, do you mean the blower fan that blows air through the vents, or the radiator fan?

 

How long has it been doing this? If just recently, have you had any maintenance or done something that might be related?