Does this one have the ISA board in it? If it does, unplug it, clean the edge connector with a pink pencil eraser, clean the eraser residue well, then plug it back in.
I had problems with my system until I took the boards out and scrubbed 'em well with dishsoap and water then drying them in the oven. Just be sure to take the battery off first!
Mine were coated with tar and nicotene crap from years of sitting in a bar. All of that had to be cleaned off to make it reliable.