cutting out and general misbehav'in

[studeclunker]

I have a 350 in my '66 Camper Special that isn't running right. I'm not much of a gearhead and am having trouble wrapping my brain around this one.

The truck runs fine up to about forty or forty-five miles an hour. Once it's hot, forty and under. After that it's popping (not backfiring just a popping noise) and kicking(cutting out?) and generally misbehav'in. Also, I have no power going up grades. We have a 6% here that a pickup with a 350 should be able to do fifty or sixty climbing it. This truck struggles at forty.

A friend of mine says this is a carburetion issue (being he's a gearhead and I'm not, I generally listen to him). However I would like to avoid the annoyance of spending the cash for a rebuilt carb only to find out this really wasn't the issue. Yeeeears ago I had seventies vintage Chevys and one of them had trouble with that little cylinder in the valve cover that has a hose attached leading under the carb. I believe it's called a check valve? That was nearly forty years ago though and my memory is rather suspect.

Anyway, any ideas anyone? The problem is getting progressively worse.

Funny thing is, even as bad as it's running, the truck is getting decent mileage.