I had a simillar problem with mine, it would only working properly if i put my foot right down, when i backed off it would cut out, idle roughly, sometimes it wouldnt even start etc.

I spent the whole day with my brother looking for the problem, in my case it was a collapsed pipe.

you may have already checked it but find the breather box which is behind the exhaust manifold on engine, there is a pipe which comes off it at the top then runs over the gear box and into the inlet manifold.

where it goes into the manifold i found about 6 inhces back from the manifold the pipe had collapsed and closed it self off. probley from old oil and oil mist etc. Anyway i replaced the pipe with a new one and it started first time and ran like nothing was ever wrong with it.

it is hard to spot if the pipe has collapsed so you may have to feel the pipe to see if it has. hope this helps in some way.