Small Engines (Lawn Mowers, etc.): Not Cutting, spindle bearings, clutch linkage

Question
I have a TROY-BILT BOLENS 20 HP lawn tractor (Model 13076-GTX 20).  I bought it New Dec. 8, 2001. It cut great, this year it isn't cutting very well, my grass is only about 3" high and in order to cut I have to go very slow. If I try to cut at a normal speed, the blades just stall, I have to lift the deck in order to get it going again. The engine also slows down. It's like the blades aren't turning fast enough to cut.

Answer
If the grass is 3" tall how much are you cutting off?  At 3" it should be 1".  

I take it that the mower preformed well before this?  What changed?  The mower, the grass or the operator?  If not the grass or the operator then it is the mower.  Has proper maintenance been done on the deck?  Clean out underneath replace belts and blades as needed.  If all this is done then  check spindle bearings and spindle shafts.   Are the bearings tight?  Is there stuff wrapped around the spindle shafts? Remove the belt and check the condition.  Inspect the belt for wear and cracks.  Is it routed properly?  If there is an electric clutch is it adjusted properly?  If not is the manual clutch linkage adjusted properly.  If all this is fine and you find nothing wrong look to the engine.  Had the oil and air filter been changed ar proper intervals? If not then you could have excessive engine wear.  Check the compression or even better have a dealer do a leak down test on the engine.

Somewhere along the way you will find the issue.