Idle revs drop on high beam
Weird problem been around for a while and I had given up trying to solve it. From time to time the idle revs, when stopped at the lights, would drop (car shuddered) for a brief period then went back to normal then drop and return etc. Been doing it for ages and wasnt a big deal. My mechanic can fine no codes or faults. But tonight I wanted to make sure my high beams were working so at home before switching of the engine flicked on high beam. In Drive, the revs dropped and the car shuddered. Low beam rev back up and shudder much less but still there a bit. Replicated over and over. In Park with the revs raised no noticable difference. Any clues why switching to high beam in Drive would change the idle revs/smoothness?
rev@IRC. EB 3.9 auto. VDO head unit, Focal and Sony speakers, tinted, mags, pacemakers, hi-flow cat, 2.5 inch exhaust, K&N, slotted.