Hy gearhead695456, it sounds like the oil pressure to the lifters is being interupted somehow. If you're sure your oil level is correct, then I think something might be blocking the oils return path to the oilpan. If all your lifters are "clacking", then you don't have a stuck "one". Take the valve cover off and make sure the drain back passages are not clogged up, there should be a few places for it to drain back down, if memory serves me correctly there should be some drain back holes behind the side cover as well, so take that off and check in there also. The fact that a new pump did not solve your problem says to me that something else is wrong, hope that helps.
