
I, too, have a XC68882RC50 math coprocessor, which I installed
succesfully in my Mega Midget Racer (clocked at 33 MHz). I have tried
clocking my FPU at 28 to 50 MHz and it all worked just fine. I have a
MC68030-33 CPU. I don't know why my FPU has an XC (my original 33MHz FPU
was label MC68882-33), but it seems to work fine on my system. Maybe you
just have a bad chip.

Stefan