I'm not saying don't go for a Piper cam but if you're looking for a wee bit more power then a cheaper option is to fit a 3.9 camshaft. They only cost about £45 for the camshaft instead of over £100 and for the rest of the kit you can just use standard parts. Make sure you get a new timing chain and gears as well though!
Oh, and you'll need the correct bolt for the end of the camshaft as they changed the thread on the 3.9 cam compared to the 3.5!! :roll:
If it's a carb engine then with the Piper setup you'll need to look at the fueling as well, ie: getting the correct carb needles to suit the cam.
I'd also have the heads off at the same time and give them a good clean, check for flatness (they may need skimmed) and re-seat all the valves.
I had similar problems to you and it was a burnt out valve, so always best to check.