When I go to engage the blades the engine dies. If I immediately disengaged the engine recovers. My seat switch seems to be working fine - if I stand up it cuts out and won't start unless I am in neutral, seated and brake on. However, one of the blades does not rotate well.. it's stiff and doesn't turn as freely as the other... could a blockage, grass tangle, etc. be slowing the blade down enough to cut the engine out? It's only been used about 25 hours and was working fine when stored in a hay barn last fall. The belt is properly positioned and the tension seems to be fine when engaged.

I would look for grass build up under the deck .
Check for bad idlers on deck.
Is this a manual pto , or an electric one.

It's a manual one. I'll get it to the garage tomorrow and check out any build up, etc. Is it advisable to take the blades off and check for gumming up around the shaft? Thanks for the help.

Could be the bearings in the spindle ,
Never heard of gumming up problem, but there's lots of things I've never heard of or seen.
Good Luck DG