Stuck in highest gear

I have a 91 grand prix gtp with about 320,000 km on it and it just dont seem to want to work right. I figured out that the problem is every so often, only after its turned on it shifts into the highest gear and runs like that the whole time. Sometimes it kicks out. Sometimes it doesnt. When I go it slowly gets up to speed seeing as how it is stuck in a tall gear but I can take it on the highway and it will not shift. It just stays in the same gear and goes about as fast as it can. Even when I shift it into 1st or second(this is an auto by the way) it still does it. But I think I actually have to shift out of those. If anyone has an idea about this let me know please.