1996 Oldsmobile Cutlass lifter knock?

Hi. Here's the thing, my 96 olds 3.1 has what I figure is lifter knocking. The weird thing is that under normal driving, the knocking all but disappears. At around 50 - 60mph, the car sputters like there is a drop of water in the fuel. I can let of the gas and then push it again and the car runs smooth for a few seconds before bogging down again. I can also floor the gas pedal to the passing gear and it will run fine for a few seconds before it starts sputtering slightly again. The whole time it sputters, the knocking becomes loud again, but as soon as the car smooths out the knocking subsides. I have given the car a complete tune-up, including changing the fuel filter, and have put a couple cans of SeaFoam thru the tank and into the oil as well, but nothing is helping. I am at a complete loss here and cant think of what could be the problem. Could you plese help? Thanks for everything.