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.

Colin Birch
August 2015