Why is my g20 6.2 diesel getting stuck at 25mph(wot)

My 1985 Chevy g20 6.2 diesel it's consistently not letting me go past 25mph unless I go down a hill it's then shifting into the gear after 25 mph and I can drive it in that gear the whole way to wot 85mph. But obviously if I come to a stop. I will be stuck at 25mph. Its a 700r4 with 198k always maintained as far as fluids and what not. Took across country no probable but when I returned my instrument cluster shorted out had to replace the board on it . It still blows fuses but not as instantly as it was if I'm setting still I can throw a fuse in the gauge fuse opening and see how much fuel I have however when I start it up and put it in reverse of turn the wheel it blows the fuse. Not sure if there's a relation to that and the getting stuck at 25mph but thought I'd add that in there. Please help thank you!

So in a more simple way it is that it is stuck at 25 on flat roads and up hills wide open but if I have enough of a hill to make my van drift catch 36mph til shift into the next gear and I can hold that gear as long as no cars or stops get in my way but around here misty city so it's always less then 36mph.