I have a JD 325 with a 18 hp v-twin engine. Tractor runs great when mower is not engaged. Sometimes when I engage the mower, it will run fine for a few minutes and then all of a sudden it will start losing power/run rough and I will have to slow to a crawl to prevent killing the engine. Sometimes it will run poorly for several minutes, then run fine for a while, and then start running poorly again. Often when it is running poorly, the smell of the exhaust is more prominent. Again, engine always runs great when mower is not engaged. I have not been able to determine what environmental conditions are causing the problem.....it's very unpredictable. Any help with what is going on here would be much appreciated. Thanks!!!