I had my MK1 shut down (lock up) a few times while driving this summer. The outside air temp was over 98F and a 400hp motor with headers was within 14" of the unit (no insulation on the firewall yet). There are pics in the install section. There's a vent (1949 airconditioning) directly over the unit, which was closed because I hadn't figured out the linkage yet, however buy opening it about 1/8" the problem was solved. It wasn't caused by airspace around the unit because there's 7' space around it; it was the ambient under dash temp.

On a side note: I'm totally amazed at how well these units work, I have constantly beat the hell out of the unit (temperature and vibration wise) and it just continues to flawlessly play music.

AL