i dug out my old JP-8000 synth the other day. After having to replace the battery and doing a factory reset and then resetting the ribbon and pitch bend wheel, i hooked it up to cubase 7.5 and created a MIDI device for it with the MIDI device manager and then set it up as an external instrument. I can now create an instrument track, select the JP-8000 and bam, im good to go.
it works flawlessy.
All i can say is that the issues i listed in my OP (apart from the last one which has now gone anyway) have miraculously gone. no freezing, no crashing. It would seem all the issues i was experiencing were to do with my Akai MPK25 controller.
I never thought i would be using proper MIDI cables again 