Take the deck drive belt off and start the mower, and engage the pto. If it stays the same, then check the belts, pulleys and spindles for binding. If it bogs down with the deck belt unattached, then look at the pto clutch or other engine related items.
It could also be the possibility that you are running on one cylinder and are not making the required power to pull the deck.
Check your engine and make sure it is firing on both cylinders. Pull 1 spark plug wire at a time, then put it back on.,,,all this while the eng is running!Bump
Was this ever solved? I am having the same issues with my mower now as well. Thanks in advance.