Also sounds like it could be fuel foaming. We recently had the same thing going on with an MDS. Ran fine n the ground, get it in the air and couldn't get any reliable rpm out of it. Solution: took the tank out of the wing (Sledge 540) and mounted it to the side of the fuse behind the engine, and problem solved. It was too tight in the wing to get any foam around it. I've got 3 OS .40 La, one .46LA and two .65 LAs and haven't had that kind of a problem with any of them.
Randy