Frustrating B&S 17.5 HP surging and dying problem

Hello all, I have a Troy Bilt #233644 riding mower. The engine is labeled 6BSXS5012.VP and 276470. When I first bought this (used) mower, it ran well until I engaged the PTO. A Sea Foam cleaner treatment fixed that for a couple of weeks. Now, the problem is back and is much worse. The governor rocks back and forth quite a bit, I have attached a picture of the parts that move constantly. I am extremely frustrated and cannot afford a new mower.

So far I've done the following to no avail:

• replaced the fuel filter
• replaced the carb with a new Briggs and Stratton replacement
• ran it without an air filter to see if that is the problem
• ran it without the fuel filter to see if that is the problem
• added carb cleaner to the gas again, the gas is 89 octane, pretty fresh

Frustrating B&S 17.5 HP surging and dying problem