Crashing During Recording

I have been having some trouble recording onto the MPC500 since buying it second hand a few weeks ago, I wonder if anyone can help me?

Whilst recording a sample, either from a line in or from a sequence in the MPC, the system will occasionally stop recording and freeze up completely, and the only way i can make it respond again is to switch it on and off again. Even stranger, sometimes it will make a series of bizzarely musical glitches which i almost think is it scanning through every sample on the program very quickly.

I think the problem has only begun since i installed a 256mb ram extension. Is it possible this is what is causing it? The specification is as recommended in the MPC1000 thread, but it isn't akai brand.

I know this is very hard for anyone to help me fix without having the MPC to hand, but if anyone has experienced the problem before and could point me in the right direction that would be great. Are there any steps I should be taking to improve the workflow whilst recording generally?

TL;DR: The MPC is freezing whilst recording, is there any particular reason this should happen, and is there a way I can stop it from happening?
thanks alot.

It's possible it's the card. Have you tried taking it out and seeing if the error still occurs?