Something comes to mind about extensions conflicting. I think I remember situations like that happening back in the days of system 7 and 8 where I'd install something new and everything would go wacko and I believe that is one example.

If you don't have it, pickup conflict manager, install and see if it can find anything that is conflicting wit the Jackhammer driver.

My other thought is do you have the latest driver for it? Maybe it was only meant for system 7?