belt to tight, gummed up drive pulleys, dry gearbox, weak or ancient motor.
The seat safety switch is not working properly correctly. If the safety does not sense that there is anyone on the seat then it will shut down the mower. This keeps the mower from running over someone if they would happen to fall off while mowing. Or if the safety is working you may have bad bearings on your deck and that will also stall your motor, or something jammed in the blades.
Normally you will start to hear a loud grumbling sound from underneath the mower when the blades are engaged. With the mower shut off, release the tension from the belt for the blades. Raise the deck so you can grab the blades from the bottom. You should not be able to rock the blades from side to side or up and down. If you can then you probably have a bad spindle. You can do this for wach of the spindles to check them all.
I would suggest running down to your local auto parts store and picking up a timing light. Once your ready to time the blades, jack up the mower, start it, and engage the PTO. Now crawl under the mower deck and shine the light up there.
Firstly, to sharpen the mower blade, remove it from the lawnmower and get any rust off with steel wool. Clamp the blade down and run along it a file. For badly damaged blades, it would be best to take it to a professional.
Assuming the blades are rotating but not cutting grass, the blades may be very dull or they were replaced upside down when removed for sharpening.
if you are sitting in the seat and looking down at the deck the blades will or should be turning clockwise, although in this position you cannot actually see the blades. pay attention when installing the blades not to turn them upside down. the cutting edge should be turninh clockwise on all blades.
Maybe you should tighten the drive belt or it could be the PTO clutch is slipping.
It is a lawn mower that is set up so that the grass, once cut off, is chopped finer by the blades before being left behind. The mulch is supposed to be so fine that it drops down between the grass and helps fertilize the lawn, breaking down quickly.
When mowing it will idle up and down because of the amount and thickness of grass you are cutting. This is because the blades will struggle cutting thick grass so the engine needs more revs to keep the blades running at a constant speed and that is also why your mower may slow down because it has used the majority of revs to cut the grass and their are not many revs left to keep the wheels spinning The up and down revving could also be caused by a dirty/clogged air filter.
Disengage blades, push down and/or lock it and turn the key. If nothing happens, the most common parts for failure are the key switch and the solenoid.
One of several reasons: (1) the blades need sharpening; (2) you are trying to mow at a higher speed than is required; (3) one or both blades are installed upside down; (4)one or both blades are bent. My best guess is (1) and/or (2).
Do your blades spin? Maybe need new belts or something jammed up and when you turn it on, it is straining the engine. Make sure the thing is off and battery disconnected before you start fiddling around under the deck and engine