1993 GMC c2500 automatic transmission question

[RoedersGarage]

Hey all I have a 1993 GMC c2500 witch has 80,000 miles on it 2wd 5.7 and I think a 4l80 but im not sure and has had a camper in the bed most of its life. Campers out now and I am trying to figure out the transmission issue. 

 

When driving in town with only going up to 60mph for not to long its fine shifts down when I give it gas and has firm shifts all 1 2 3 and 4 and reverse works too. And My in town/ back road test im not easy on it im trying to make it screw up so I can figure it out and it acts fine. 

As soon as I go about 5 miles on the highway going 70 or so for a longer period of time than in town, If I have to give it more gas to go up a hill It will shift down then loose 3rd and 4th gear. And act like its in neutral untill I put it and second and pull over. Shut the truck off and go again and it acts fine untill you do the same thing and shift down and your in neutral again. 

Any thoughts on this is much appreciated this truck is super clean and needs to be flying down the highway again! Thank you. 

[Mikebtte]

Kind of a weird issue, possible shift solinod issue, there are a lot of kits to replace them all.

You could try removing an cleaning the valve body in case it is contaminated.

Install new filter and fill with TES-295.