Scag freedom Z with 27hp briggs

Was having issue getting the electric PTO to engage. Figured it was the switch so got a new one. At the same time my battery went dead, got a new one of those too. The new switch worked intermittently and drove me crazy, but the new battery was fine. Cut lawn about 2 times and found that when the PTO won't engage, I have to tap on it, with the PTO switch engaged (and a heavy rock on the seat) then I can jump on and mow. It will not engage if I don't tap it. When I first start the mower everything works fine. Thinking heat might have something to do with it. Just now everything was working fine and stopped to empty bags. When I tried to start the mower up the battery would barely turn the motor over and then it died and started making the clicking noise with the key turned on. FML. The mower has 300 hours on it. Any suggestions where to begin the diagnosis this.

Jump start the engine then do voltage test at the battery.
Should be 14V if the battery is flat.
Check the battery leads at both ends.
I like to polish the ground bolt and also strip the paint from around the bolt hole.
Then paint over after tightening with liquid electrical tape.