I have an '04 600 SDI that had the exact same symptoms a couple years back. Once I got to mid range it would cut out, almost as if it was loading up. Turned out to be a nightmare! I tried several things on my own... cleaned exhaust valves, changed fuel filter, plug caps, reeds & boots, relays, etc. Nothing seemed to work. I was convinced it was a fuel problem but it was my first fuel injected sled so I was lost. Finally caved and brought it in to the dealer. He determined one of the fuel injectors was plugged (I believe two of them are for mid to high range?). Got home, took it out for a ride only to have the same problem come back! Took it back in again for more diagnostics... Took several hours of trying different things and ripping back and forth in the field out back to finally get it running right. I think he ended up changing the timing, along with some other electronic settings, but unfortunately I can't tell you for sure what fixed it.
I tried to get the $220 back for the injector but he claimed it was bad whether it was causing the problem or not
I now run fuel injector cleaner through my first tank of gas every year just as a precaution - not sure if this is a dumb idea or not but problem has never come back.
Good luck with your diagnosis.
