Is this something new and out of the blue or did it build up over time? If you are getting a good "snap" spark at the plug, we may have a fuel starvation problem at or near WOT (wide open throttle).
Float level should be checked against specs and set correctly to eliminate that possibility. Here is a good test to determine if it's ignition or fuel related:
Get yourself one of those plumbers sweating propane set-ups (like a Burnz-o-matic at home depot) and connect a vacuum hose to the valve assembly. So, now we have a propane bottle with a valve and hose attached. Turn on the valve and propane comes out the hose. Now, rig the set-up so you can introduce a little propane into the air cleaner assembly WHILE RIDING YOUR RIG AS THE PROBLEM OCCURS.
After warming up the engine to operating temp, go on a test drive and get the engine to act up under load. At the point the engine falls on it's face, slowly open the propane valve. This will artificially enrich the air/fuel mixture. If upon enrichment, the engine 'picks up' and runs regularly, we have a fuel starvation problem. Either caused by a delivery (from tank to carb) or internal carb problem with the main metering circuit not feeding the hungry engine.
The propane tool can also aid in diagnosing a vacuum leak at the carb gasket. As propane is introduced to the area around the carb with the engine running, a lean engine will pick up and smooth out. This would indicate a vacuum leak.
To diagnose a No Start condition, check for spark in the normal fashion. If you get a HOT spark, place the propane hose in the carb throat or air cleaner housing and start the propane flowing lightly with the bottle in the upright position. If the engine now starts and runs on propane, we have a fuel problem. To confirm, while the engine is running, close off the propane. The engine should die if the carb is not functioning properly.
To see if the idle mixture is set too lean on the carb, warm up engine, introduce propane slowly into the air cleaner with engine running. If the idle picks up and increases, we have a lean condition at idle and the idle screw can be adjusted accordingly to optimum idle quality.
Hope this helps with your diagnosis.