No... only the 68040 CPU halfen the clock speed given to it, so a 50Mhz crystal is what you need to clock your 68030 to 50mhz.
As you have troubles with the card and knowing there's overclocked components on it I'd first remove the FPU to rule out that. Also I'm sure you can get hold of a different simm just to test. If you decide to go for 50Mhz you'll need to replace the FPU for at least a 40Mhz version also or it'll burn soon (I also suggest some kind of cooling on both the CPU and FPU, if atleast an as big as possible peice of aluminium sheet will probably still fit underneath without any big mods to the computer).
