How to fix LT160 John Deere Lawn Tractor running ruff?

today while mowing with my JD LT 160...running smoth, full tank of gas...I pressed the backup switch to backup while blades were engaged...engine began to run ruff as if choking on too much gas... I have replaced to fuel filter, cleaned air filter...but there is still a problem.

Interesting note...runs more smoothly when I have throttle at 1/2 speed, sputtering and uneven rpms when throttle is on high setting OR blades are engaged. Please advise.

scott
November 2011