that is interesting, i believe you are right with the lights dimming when it idles roughly, have you tried testing your alternator? just a conclusion im jumping to...maybe your alternator is going bad and the battery voltage is low because its not being charged up? reason i say that is because i had a navigator with a sub system and during the day my alternator died (i did not know) and when dusk came and i turned on my headlights i had a semi rough idle and all my lights were dimming in and out(probably all the demand was on the big battery)..it did start tho....so it could be?
just a shot in the dark
i replaced my alt and all was good again... hopefully its an easy fix for you and nothing internal :/ best of luck