Denny,
Just another vote here for ignition. I had an old Mercury that acted a lot like you're describing. I could rev the snot out of her in the driveway, but take her out on the highway under a partial load and she'd buck and spit unless it was wide open.
The previous owner had just had a garage do a complete tune-up so I didn't think ignition. Went through the carb, filter, pump, etc, all to no avail. Finally after a lot of suggestions, I dug into the distributor. To make a long story short, even though it was brand new and I couldn't see any cracks or carbon traces, the cap was at fault. A new cap and she ran like a champ.
Bob
