The interior and exterior lights in my 2004 Tiburon seem to dim periodically after driving for a bit. People think I'm flashing my lights at them. Once it starts, it seems to keep it up. Also, when this happens, my CD player/radio cuts out, too. It's like listening to a really scratched CD with an accompanying light show. Of course, this never happens for the mechanics when I take it into the shop. They told me to watch for things that trigger it like rain or going over bumps but it seems pretty indiscriminate. Eh?

have you had your battery and charging system checked out properly?

could be a bad alternator

Do they ever go out completely? It could also be a motherboard issue.

They never go out completely. It's like a hiccup.

I've had some strange problems with Tib's. I had one that the radio would go blank, and the check eng light would come on at times, even with no DTC. It would act-up especially when operating the power windows. We traced it to a high resistance connection at the heavy wire from the positive battery terminal, to the engine-room fuse/relay box. We found it by turning on all of the vehicle loads, then measuring voltage at various points. We found a voltage drop from the pos-cable to E/R relay/fuse box wire (about 2-volt drop). I made a new wire with 8-gauge wire, and new ends, and it fixed the problem.

Update: It only happens with the A/C on. If it's off, no problems.