I have a 1998 20 hp. John Deere 345. After mowing for some time ( an hour or so) and putting it in a good pull (tall grass or up hillside) the engine will bog down, I can choke it and it will rev back up for a few seconds and then it will bog down until I choke it again. This will continue until I get to an area where there is less power needed.. The engine is making a cackling sound. While mowing in shorter grass and not requiring a lot of power, it runs fine! I have changed both spark plugs and fuel filter. It then ran fine for about 30 minutes in same work load and then started again. I also notice that after shutting it off for a few minutes and starting it back up it will do the same thing just driving it across the lawn (mower not engaged)..The tractor has about 1000 hours, uses no oil. Anybody got any ideas?