Sorry to bum you out, but I'd dump the D-70
if I were you.
Sounds like your midi problems are caused by a simple midi loop (is your midi thru set to thru or out?), but I can tell you from personal experience that the D-70 was THE most bug laden piece of crap I ever owned.
I used Midi-Quest as a librarian and could never edit patches (the D-70 would lock up), only performances. I thought maybe it was the software, but after talking to them AND to Roland, it became clear that the D-70 is pretty screwed up midi-wise.
You can read the entire gory story in my review at Harmony Central (I can't recall the link, but look at the synth database, under Roland, then under D-70).
Even the techs at Roland admitted that the D-70 had some pretty severe problems.
I even had an OS ROM replaced and that didn't help at all.
If you really want to continue this unnecessary self abuse, I would suggest reviewing the channel assignments in the performance your'e trying to play back.
Chances are it has a bunch of programs all set to channel one, if you go in and change those to other channels maybe you'll hear the other parts.
Another thing to keep in mind:
Like most older (pre 1996 or so) Roland gear, you only have 28 voice polyphony, and most Roland patches use 3 or 4 oscillators.
A three oscillator patch is only going to give you 9 notes total, so if you are trying to play back a performance and you've got more than say 10 notes or so going at once, you're going to get voice stealing.
The D-70 processor has a hard time dealing with this in Performance mode anyway (even if you aren't maxing out the polyphony), notes will tend to stagger and play late, so I wouldn't depend on the D-70 for more than one or two parts at once.
I sold mine back in 1995 (after buying it new for $1250 in 1993) for $800 and I've never regreted it...
Jay Storey