Once I've been driving for a while (30 miles in Florida heat, usually), after cruising at a set speed, I can give the car a little more gas and it shakes pretty bad. Almost sounds like the engine is knocking. The thing that's got me is that it only lasts for a second or two, then goes away. Immediately after I can gun it and it's completely smooth. Wait about 30 seconds and it's shaking again.

Any ideas?