The 7100 has the funny DVI-like all-in-one port designed for the AppleVision monitor. ADB, the graphics signal probably power too all go through it. My 8100 has an adapter that allows me to plug in a normal DB-15 monitor cable to it. Get a hold of this adaptor, if you don't already have it, and try plugging your monitor into that. If you don't have one, I've one one spare somewhere.

Another idea: do you have any NuBus cards? If so, try these.

I'd give your mobo a recapping anyway. It's 21 years'old now. It'll need recapping sooner rather than later. How's the PRAM battery btw? You might replacing it.