Internals are just that...the "guts" of the fuel pump.
If you have the tools and the skill to take the actual fuel pump apart and replace the guts of the pump without damaging anything or screwing up the install, you can save yourself some money by not purchasing the actual completed pump (in the same way that you can buy the "guts" of a computer and assemble it much cheaper than buying a pre-assembled computer).
I ordered the PTP pump assembly a couple of days ago...the Hitachi upgrade. Remember the pump is $300, but there is a $350 core charge added to that. They charge you $650 in total, but refund the $350 once you mail them back your old fuel pump. Or you can just get the internals and rebuild your pump yourself. The "maxed out" internals I am clueless about...I have no clue if they work or if anyone has them. They are pretty pricey, though, I know that...which is why I skipped them. I had the original pump fail on me, and the set of aftermarket internals fail on me (that company is out of business now I guess). So this is my second attempt at correcting this issue. I do not, however, have any input as to the PTP unit because I haven't received it yet.
There are a lot of reviews, good and bad, about most of the aftermarket (and OE) pumps available. This seems to be the weakest link for our cars, and it is taking time for companies to fully solve the problem.
I would suggest getting a DashHawk or something to log the fuel pressure (or at least monitor it) because there are other things that can cause what you are talking about. However, if your pump is delivering less than 1650 psi at WOT then it is no good.
