1992 Ford F-150 engine is sluggish "sputters" when

NOTE: Engine only has about ~70-80k miles on it, the original owner replaced the engine at some point. Engine is a fuel-injected in-line 6cyl 4.9L.

After sitting for 1 month without being driven, this truck started having performance problems, especially when you floor the pedal. I try to get it out on the highway once a week to keep it running well, but unfortunately I let it sit for awhile.

When you hit the gas, you get the initial "surge" of acceleration, but if you keep on the gas, the engine sort of "sputters" and you lose power. I don't think it is knocking, but I could be wrong.

To make it up a bit hill, you have to repeatedly floor the pedal and release, floor and release, etc. Last year, I could not get this truck to start. I replaced the spark plugs/wires and distributor and it's been running great since then.

When you have it in park and rev the engine, everything sounds normal. It's just once you get it in drive and hit the gas. I took it out onto the highway for about 30 minutes running at about ~70-80mph, but this did not seem to help much.

Here are a couple of pics of the engine: