Probably a combo of both. You know how I said above that there are some cool design techniques to get the old waste gases out of the cylinder? Well those techniques only work over a very narrow band of RPMs, under very specific air/fuel ratios. Your problem is even worse because you're in an aviation setting. Your engines are experiencing very different air densities, and this is causing the air/fuel ratio to change drastically compared to what they've been tuned for. If they get too rich, they stall. Too lean, they explode.

The leaky oil? Yeah, that's probably just a two stroke being a two stroke. Due to the design, they aren't "oiled" like a car, you instead shoot the oil in with the gasoline in the injection cycle, and it just kinda seeps in and around the engine. That's the main reason why they're dirty - both literally and environmentally.