Definitely re-seat the SIMMs, when you have them out, give 'em the pencil eraser cleaning of the teeth treatment while you're at it.

Is your Quadra set for 32bit addressing? ISTR this being the only way the Quadras ran, but check the memory control panel to make sure anyway after you get it to boot. While you're at it, make sure Virtual Memory is off.

Can't think of anything else offhand . . . sleepy . . . xx(