Glad to see you fixed it. I was going to suggest getting a simple bike speedometer. The way they work is a magnet gets mounted to a spoke and then as it passes by a sensor a pulse is sent to the computer. You simply tape the magnet to the PTO shaft and find a way to get the sensor close enough to read. The speed will mean nothing but if the number didn't drop when the mower was slowing down it would have isolated which was the problem, the tractor or mower. It's not a perfect way but would get the job done without spending much.
I'm glad to see it was easy instead of going the hard route! Some mistakes were made, but I learned a lot about the tractor with everyone's help. When I pulled the pulley covers to observe the belt I thought I'd see the pulleys slip under a load if the belt was loose, but that didn't happen. Your idea may have come in handy at that point. I tightened the belt tensioner a quarter of a turn and noticed when I engaged the PTO, the sound of the blades seemed to sound faster and the slight squeal on the initial startup was gone.
Took it out on a test run and when I hit the thick grass... She the mower deck stayed steady... You could hear the tractor and deck come under a load mind you, but it was in tandem, so good to go with some final adjustments.
One thing I reported incorrectly... The PTO was bogging down, when in fact it was the mower bogging down. That's what it sounded like and early on and I would have sworn on it. The finish mower drive shaft is covered by the sleeve, so I could not actually see it. My neighbor went off that input also so we missed the obvious early on... I can easily understand why folks went deep into the fix. PTO vs. mower deck is a big difference. That's on me...
The PTO handle did pop out of gear several times over the past month. I did not imagine that. One thing I noticed, it seemed to happen shortly after the mower bogged down and it was 100 degrees outside and several hours into mowing. So... Who knows, I may still have an issue with that. The other day it was 80 deg. and after 5 hrs. Mowing she didn't do it once.
Learned a lot and didn't mean to upset anyone... Hope this feed helps someone down the road. Noticed someone else had a similar issue years ago, but never posted what fixed it. To me, that's a shame. We all make mistakes and never easy to swallow your pride, but rather do that than lead people astray,
Again thanks for EVERONE's support. Next time I only report facts. :0)
Also received my repair manual.. I will use it to help folks whenever I can.
Cheers,
MAC