Pita!....
Man, that's like the biggest bummer in the world. It's like you have a problem, every time to you it, you have the same problem but as soon as you carry it to the shop, it seems to work fine!.........then of course, you have to pay them to tell you, it runs great.....
I don't mean to poke at the DI but it's my (and many members) worst nightmare. It's the hardest 2 stroke there is to troubleshoot. But, if you get one that runs good, it's faster than the RFI models.
I don't think it would be a sensor. If so, it would register a code. Did you actually witness the shop put the ski in a test tank? Just curious since I have this big problem with trusting that these guys do what they say they do, then charge you an arm and a leg, just to know, when you bring it back again, they will tack on more charges.
The DI system using the compressor (which it seems you already know something about) and air injector, fuel over the top, might play a part in your problems. I would test the compressor output to make sure it's working properly. There is a solenoid that bleeds off excessive pressure. Because the compressor is mechanically connected to the engine, it responds to the speed of the engine. So, pressure is regulated by a solenoid that bleeds it off if there is to much. If this sticks open, then you haven't enough pressure to atomize your fuel for full power.
The compressor system also works your RAVE valves. If you have a leak in one of your air lines, you'll also loose pressure, causing the system to limp because of not enough air for combustion.....
I believe your problem is going to be related to your compressor. Don't take it back to the shop, it's eventually going to cost you big bucks. See if this is something your capable of doing. Inspection of all hoses would be the first thing I'd suggest. The solenoid can be tested by jumping 12 volt power to it. You should hear it click open, then shut when power is released. If this is all good, then you might hook up a pressure gage and see what kind of stable pressure you get behind the compressor. :cheers: