1998 Sentra SE Problem

Alright, so a couple of days ago I bought a 5-speed 1998 Nissan Sentra SE. It runs perfectly fine unless I really punch it, after about 2/3 throttle it starts sputtering and the rpm's will fall. It does this wether I'm in gear and driving, or parked in neutral. Also It won't go past 5k rpm's, even in neutral.

Tomorrow I'm going to put in some new spark plugs, somebody told me it could be dirty fuel injectors, and that I should go buy some sea foam or something, but I read in the sticky that I should not use fuel injector cleaner on this car...

Any ideas?

Thanks!

How many miles? Original fuel filter? Unless it's known to be fairly new, I'd replace it. (not too difficult). Then, you could measure the fuel pressure, see if the fuel pump is putting out (should be ~34psi at idle; ~43psi if you pull the vacuum hose off the fuel pressure regulator).

Also, I believe there is a rev-limiter to keep the rpms down in neutral.
Hope this helps.