Well, my first thought was like retiredoz posted, a binding gearcase due to the driveshaft shock absorber binding on the upper bushing/seal housing.
But, I would need to hear how the engine "bogs and dies"…Does it misfire, run rich, drop a cylinder while it is bogging/dying? Or, does it seem to run normally, just lose power and die…The shock absorber is not to blame if the engine misfires/runs rich/drops a cylinder while dying.
Anyway you can post a video of the engine misbehaving? You mentioned the engine is hard to start after it fails. Is it hard/stiff to pull over at this point?
Please do not pull the gearcase off based on this post, do the simple things first. What the heck did the repair shop do with the engine for a month? Did they offer any advice, or check anything? I am assuming they saw the engine misbehave when they ran it in their tank, maybe not though.
Again, simple things first. One thing you can do is to drain/refill the gearcase. If you see signs of water in the oil, then the shock absorber may be suspect. But, just because there are no signs of water in the lube does not mean the shock is not at fault.
Hopefully, I am totally wrong and your issue is a simpler one….Carb/fuel issue or ignition problem. Have you drained your fuel tank, perhaps there is water in the tank. You might try using those inline neon spark testers on the plug wires so you can watch them when the engine is misbehaving to see if you are losing spark on one of the cylinders…
The engine might be overheating when you throttle it up also. Does it appear to get hot when it misbehaves and stalls?
Finally, I would keep the fuel/oil mix at 24:1…