Engine misfires under load when hot?

I have a Castle Garden sit on Lawnmower,
My problem is it misfires under load after about 1/2 hours work. It is a Hydro drive transmission with a forward & reverse lever pedal.
When it starts to misfire in movement, I release my foot off the forward pedal and the revs will rise immediately to full revs, as soon as i put pressure on the forwards pedal the engine starts to pop and die out, if I release the pressure on the pedal the engine revs come back to full revs.
I have carried out a full service changing all filters and inline fuel filter. I have changed both coils for new ones.
The engine runs perfectly under full revs whilst not moving, but as soon as I try to move the engine breaks down again.
If I leave it to cool down for awhile its fine again until it gets hot, then starts breaking down again.
Is there any link between the Hydro drive and engine.