STUCK MIDI notes and System Lock up

I’ve just upgrade to Cubase 7.5 using windows 7. I am also using Steven Slate Drums 3.5 (32bit) and have just added Steven Slate 4.0 Platinum Drums (64bit) to my system. I’m using a Triton Extreme and a Yamaha Motif to run 24 tracks of MIDI, I also have 20 tracks of audio. Here’s my problem: At the most unexpected times my system just locks up, the MIDI note stick, but the audio continues to play and I cannot shut it off. (I also have a FW-1884) The only way to stop the madness is by shutting off the entire system. ANY IDEAS on how to fix this nightmare??? Do you think the Slate 32bit drums are the problem since I upgraded to Cubase 7.5??
Should I reintalize the Tascam FW-1884??
I wasn’t having this problem very often until I just redid my recording system last week.
PLEASE, PLEASE HELP!!!
Thank you!
Gary Gelinas