It's been a long, long time, but my old Performa 600, even with vram maxxed out, was picky about what monitors it would drive; I recall my disappointment (and the Apple reseller's surprise) when it simply refused to run a bog-standard 15" Apple monitor, and I had to settle for a 14" screen for a few years. (This is back in the day when these things were newish and still pretty expensive, and I certainly did not have a spare $500 to drop on a nubus video card for the thing.)

A small (14") monitor on a IIsi should work, but in the absence of that ... does your monitor offer native 640x480 resolution? That would be one question to ask. You might also try cleaning & re-seating the system ram; since your video is running off of those ram chips, they may need some TLC before you get any further.