Please note that these suggestions are little more than stabs in the dark. Don't put more faith in them than they deserve. I'm not really sure on the details of this engine, either, in terms of its specific systems and sensors and whatnot.
Stumbling under load can of course be caused by quite a few things. One possibility that comes to mind is a fault in the MAF sensor (assuming the engine has one). Perhaps the connector is a bit loose or something and becoming intermittent when hot.
Other ideas: Plug wires? Distributor (again, if the engine has one)? Fuel pressure regulator a fuel injected engine? Intake manifold cracks or looseness or broken mounting studs?
