Could be carbon deposits in the carb. I had this happen to me before. Got to the point where it would have no power at all in certain places. The RAC man just redlined it a few times in neutral and that sorted it out.
Try taking the air filter off and pouring some reddex down the inlet. You may get the odd plume of dark smoke from the exhaust, which for once is a good thing!
Another thing to check is the carb flange (a rubber thing that connects the carb to the inlet manifold). It tends to split at the front and let air in, leaning your mixture out.