Kawasaki 23hp problem

I have an 02 Hustler z 60" 23hp Kawasaki with 450 hrs. At idle, it speeds up then slows down. At mowing speed it does the same but is not as noticeable. Last fall I replaced the fuel filter, spark plug and cleaned the air filter with no improvement. It's worse this spring, as it will die if throttled all the way back. In storage I just add stabil to the gas each year. It has the original battery! Any ideas?
Thanks gatorblade

Check for cracks in the intake manifold . Sounds like air is getting in there other than thru the carb. Check to see if the rubber fuel lines are not dry or cracked - air can get sucked in with the fuel effect performance .

Thanks Sonofadocker
I checked out all your suggestions. Even took off the carb. Things looked ok. Iv'e not worked on something this knew. My cars are 60's, mowers old except for HustlerZ. I thought it could be dirt in float bowl. I seen no bowl. There were two bolts on bottom of carb, had the #5 on each one. Close to each bolt there were two brass colored bb's. Thought this may be something, couldn't break loose. I didn't want to use much force untill I knew what they were.
Gatorblade