I once had an A4000T with an Cyberstorm MK2 overclocked to 80Mhz (using latest mask 060). It was very fast (for a Classic that is) and never had any stability problems with it. If it doesn't even boot the problem is elsewhere. It might seem a strange suggestion but have you inspected the contact between the MKII and the CPU slot ? I once had a PicassoIV fail on me and I suspect it was because of that, it worked everywhere except on the A4000T but the 4000T worked with every other card. Another suggestion, failing other attempts maybe update the capacitors on the 4000T / MKII. It might not solve anything but those tend to not work within spec after all these years.
