There are intermittent failures and these can be difficult to diagnose and repair. It is much easier when a part fails completely, and stays dead. You could have a loose connection somewhere. Another possibility is a dead spot on the starter motor commutator. Think of a roulette wheel in a Las Vegas casino. The wheel is spun up, and stops at a random location. Much the same happens when the starter motor is energized. If there is a defect in the commutator, and/or worn brushes, the starter might work OK, the next time it is needed. Or, it might not. It just depends on where the commutator came to rest, the last time the starter was used.

Try jumping 12V directly to the small terminal of the starter solenoid.
IF it cranks, you have a bad or mis adjusted- (or bad connection to)
Key switch
PTO switch
Brake/clutch switch

Seat switch is not involved in the crank circuit.

Grounds, grounds!! Please check all ground connections.
Mike

On your solenoid there are 2 small terminals . 1 will have a ground presents if all the safety switches are online and the other 12V when the key switch is activated purple I think. If you have both- maybe you have a bad solenoid. This is old enough to where corrosion at the ignition, fuse block and brake switch connectors could be present. If you can safely jump across the 2 large terminals and it cranks, that would satisfy that side of the starting system- that the engine is crankable and the starter, battery and main grounds are good.

Jumped from my truck to the starter and it fired right up. Possibly bad ignition switch?

Possibly.
Possibly a bad PTO/attachment switch.
Possibly a bad brake/clutch switch.
Possibly a bad connection.
Possibly a bad battery.
Possibly a bad key switch.

All you've tested is the starter motor.

have you been washing that machine with a garden hose, power washer or the like.. or leaving it out in the weather?

@ rcbe: hosed it off due to mud from recent rain. It does not stay out in the weather. It started after being washed off but now that you mention it, the problems did start soon after.

longlosttoo - just one recent hose washing prolly won't have caused your current problems - takes a while for serious electrical connection corrosion to build up to where circuits begin to fail. but - if you've washed down the tractor in the past - then all bets are off.
Just bear in mind L&G powered eqpt are not designed/built to withstand nature's elements, particularly moisture. Better off with compressed air or a leaf blower and mebbe a lite sponge bath on exterior just before waxing..

d120 comes up tight i take plug out it cranks over fine put plug back in and it starts new battery and starter cant figure it out

SOunds like your compression release isn't working. Lots of threads on setting valve lash. That's a good place to start.

Thanx a lot

Sounds loke a stuck exhaust vale or somehow the cylinfer is not letting the compression out and you have two cylinders with compression at the same time and the starter doesn't have enough power for that and a good thing or you might break a rod or cylinder. Take the valve covers off and look. Mine did this.