I had a simular problem: always started when cold, but hard to start when warm.
Did you inspect the jets on your carb yet? It's really easy to do. Norm's carb tear-down is very helpful. I just sprayed-out the jets with Gumout, and flushed/filled the tank and installed a good Briggs/Stratton inline fuel filter. If your fuel fileter keeps plugging-up, I would consider lining the the tank with sealer.
You should be able to see a perfect tunnel of light through each tube (jet). Only takes about 30-45 minutes to dissassemble, clean, and reassemble the carb. Being that it is so quick and easy to do, it can't hurt (unless you're worried about wrecking the carb). Otherwise, ThatsDax is sells carbs for $25with shipping.
Food for thought.