I'm having the same problem with my drive. I read this whole thread just before going out to clear some snow, as I needed to adjust the auger belt and I never received an owner's manual with my machine. A google search had this forum as one of the top items.
I was able to figure out how to adjust the belt for the auger, but not for the drive. I sprayed some belt dressing on both belts. It definitely helped the auger belt, as there was zero slippage on that. It was really loose beforehand. However, my drive was slipping quite a bit, and it was very early in the job. It does seem a little loose, but not really bad.
There was no snow falling onto the engine or the cover, nor was it raining or snowing outside. It started to slip within about 60-70 feet on my first pass. I was discharging the snow forward and to the left, in about the 10 o'clock position, if that helps any of you to further diagnose. It did it a bunch of times early on, then not as much later, but I was using higher speeds. It seems to happen more in 1st, 2nd and 3rd gear and not nearly as often in 4th and I don't think ever in 5th or 6th.
