Ok, I've had a chance to do some more experimenting with this. As suspected, the 6v supply just isn't enough. I noticed this by the LCD screen's behavior on the 6v supply, it was sluggish when updating and kind of glitchy. On USB or the higher voltage supplies it works normally. I tried a 7.5v 1A supply, and it put out 10v under the arduino load. Vreg chip got hot enough to get that burning feeling after checking its temp for 4-5 seconds. I found a 600ma 7.5v supply which I had to rewire for positive tip polarity, tried it out. That one had 9v under load. Unfortunately, vreg chip is still burning hot. Back to using USB cable for now, and vreg chip is running nice and cool with that. Might have to get one of those well regulated USB charger supplies, I've run out of suitable wall warts to test.