I am pretty sure your problem lies in the powershif module or 'brain' that is in the compartment under the steering wheel, next to the fusebox.
I have had 2 5088 tractors, and both had the same problem:
It would 'occasionally' just seem to pop out of gear, and you could get it to go again by pumping the clutch. After a while, I found out that shifting the powershift up or down, often would get it moving again quicker. It seemed to have more problems in either the odd numbered or even numbered gears (the hi or low side of the Powershift, but I can't remember off the top of my head which). There are usually one of three problems.
1) you have a powershif relay going bad. This is a cheap, simple fix, but hard to diagnose. There are 2 or 3 relays, but they all interchange. Easiest is to just buy one new relay, then swap one out, see if it works better, and if not, swap out another. Or carry it with you until it kicks out, then swap out the relay that is the hottest, as they usually get hot when they go bad.
2) you have a pressure sensor going bad. There are pressure sensors under the removable plate under your feet, one for the high side of the powershift, one for the low side. If you have only odd numbered gears or only even numbered gears when the problem arises, you may have a sensor going bad. Best way to test is to get a known good sensor, and swap out yours one at a time, and see if it cures it.
3) the powershif module itself is going bad. The factory module is an extremely overpriced unit, that will give you sticker shock, heart palpations, and feelings of being robbed in broad daylight. The reason it is so expensive, is because it is full of computer circutiry that is designed to disengage the powershift if it detects low pressure to the clutches, to prevent damage from slipping. However, over time they seem to get false readings that cause them to kick out for no reason, and sometimes get a crack in the insulation or heat sink, that causes them to malfunction. If the problem never seems to occur when you first start the tractor, and happens more frequently the longer it has run, chances are this is the problem, expecially if it was just fine over the winter, and is now causing problems when the weather is hotter. If the module is bad (and I'm betting that is the culprit) save yourself a minor heart attack, and get the Hy-Capacity non-computerized module (for MUCH cheaper). You can order them over the internet, or anyplace that sells Hy-Capacity parts. Basically, they eliminate all the computerized circuitry with cleverly wired relays that are individually replacable at under $10 each. On my tractor, the shifting might not be 'quite' as smooth as with the factory unit, but it is very hard to tell the difference. Mine has been there almost 20 years, with zero problems or ill effects to the tractor. The only thing you need to keep in mind is that if you lose pressure to the powershift unit, it will not shut itself off, like the factory unit. However, the big red 'Sentry' light on the dash will light up in that situation, so if the light comes on, you know it means unsafe pressure to the powershift unit was detected.
Hope this helps
