Yamaha DTXpress/DTXplorer/DTXtreme

I have my DTXpress II connected to my laptop (and cubase) via midi.  
When I play midi files in cubase they trigger sounds on my DTXpress 
fine, so I though I would record some of my favorite tracks down to 
the DTXpress so I can play along without having the computer on.  The 
only problem is after recording a few bars (at most) the DTXpress 
runs out of memory :(
Surely there is more room available, I only have 2 extra kits stored 
in it and the midi tracks I am recording only have 4 or 5 tracks at 
most.  Is there a way around this that anybody has found, i.e. only 
send certain events to the DTXPress ?

Thanks

Mark