Trever,
As people above have stated, it sounds like a vacuum leak somewhere.
There is a slightly dangerous but very effective way to check for vacuum leaks, using a
propane torch. Use this technique at your own risk. It does work.
Regarding a stuck valve - you could check this by taking off the valve cover, run or turn the engine, and see if all the valves move. If you have a truck 261 engine, it came with solid/mechanical lifters and a struck valve would cause more obvious problems (I think).
Some 261 engines were able to use hydraulic lifters (and I think the Canadian Pontiac 261 had hydraulic lifters). I know very little about hydraulic lifters but my guess is that a stuck valve might not be as obvious in those engines.
Also, an old-style
vacuum tester might help you find the problem.
Tim
