Some times the L2 cache on the G3 card is bad, and that certainly sounds like the problem you're having. On my first G3 upgrade for NuBus machines, some of the heat sink grease was dribbled across the cache pins during manufacture. That stuff is electrically conductive and so it was causing a short.

My G3 upgrade behaved exactly as you describe yours. It would always freeze the machine when the software loaded the cache. My G3 upgrade was a NewerTech model.

Pull that G3 upgrade, install a PDS termination card and try powering up again. Your 6100 may be fine and it could be the G3 causign the problems.