the key here for me would be its fine when its cold, but only happens when its hot...also it only happens under load....

when cold you probably havea cold runnign injector 'riching' the mixture up...when its hot you dont which would lead me to suspect a fuel injector..but not necessarily...

I would also wondeer about spark plugs or a coil on the way out...(both will ONLY show up under load in the right scenario)

for me I would change the coil & plugs (as they are cheap!) and pull the injectors and spray them into a jar and see what they look like...i would also wonder about the fuel presure regulator....

also thinking about it i would wonder about a blocked exhaust (or collapsing inlet hose), again both will allow you to run fine at constant workload but will tend to show when you jump on the loud peddel