Cutting out really bad.

Heya, I have an 86 Pontiac Grand Am.. I've been trying to figure it out, But I'm having no luck.. Usually going down the high way.. I'll hit about 50mph, and I'll start to lose power, If I continue accelerating to 60mph, The car will actually start cutting out really bad, As if it wasn't getting enough gas or something.. I'll end up losing speed till i'm back down to 40ish ( or lower ) , then accelerate and the car will be fine for a little bit ( not always ) .. Also, The other day I was going up a semi steep hill, I hit it in 4th gear, Ended up having to down shift clear to 1st gear and was moving at about 10mph up the hill, As I got to the top, I stopped giving it gas and it actually died on me..
If anyone has any idea on what could be causing this, I'd really appreciate any feedback!

Thanks in advance =)

Your engine size and how many miles on this motor ?

Tho It Sounds like you might need an overhaul !