Given you had PTO problems that likely left a pile of contamination in the oil.... I'd wager that the grit is causing problems with the unload valve in the priority pack. You can remove the pack (just under the seat) and clean all of the valves in solvent and inspect the spools and bores for scars... then reassemble. It'll probably work again for a while. Ultimately you'll need to get all of the crap out of the hydraulic system.
If this was a continous problem I might be inclined to think there's a problem with the linkage under the lift cover... but that's doubtfull given how it behaves.
The brakes aren't also coming apart and causing contamination?