So here's a weird one. Last night while I was driving my Suburban I noticed that while sitting in gear idling with my foot on the brake, my headlights would flicker and dim slightly along with the engine idle. Turned on my interior lights and they all do the same, cluster and everything. As soon as I step on the gas it evens out. On to issue number two. Whenever I use any accessory the lights dim and I get a pretty good voltage drop on the gauge. Windows, door locks, hazards, you name it. Same result with every switch I hit. This issue is persistent whether the engine is running or not. Now for the kicker. I let it sit on my trickle charger overnight and took it to two different places this morning to have the battery and alternator tested, they both tested fine at both locations. My connections at the battery are very clean and 100% corrosion free. Could this be a cable or ground strap issue? Would upgrading my big 3 cables be a possible solution? Any info would be awesome, let me know if there's some other info that would be pertinent here. Thanks in advance.