Hi there, only1divine!
It could be one of two issues from what I can tell. I was unclear if it was a surging problem or a losing power problem so I'll give tips for both cases.
An engine losing power under a load could be caused by a few things including carbon build-up on the head, valves not seating properly, intake manifold air leak, or even a blockage in the carburetor or a diaphragm that needs to be replaced. I'd suggest you start by replacing the diaphragm.
Surging can be caused by one of two things: either a dirty carburetor or the governor needs to be adjusted. You can clean the carburetor. Here's a link on how to do that:
bit.ly.
If the surging still persists after you clean the carburetor, try adjusting the governor. Here's a link that will help with that:
bit.ly. Let us know how it works out!