2003 Nissan Altima possible transmission problem?

Does it have any power up to 20? In neutral, can you rev it normally and does the engine sound normal?

if it revs normally in neutral but won't go when you put it into drive, it could potentially be the alternator as the battery likes to be continuously charged while running in order to go faster.

It "could" be a transmission problem.... you didn't say how many miles were on the car. And you didn't mention if the gears were shifting on their own or if you're getting stuck in 1st gear????

Also, if it is an automatic transmission, can you start in 1st gear and manually change the gears? If it will do this, I then suggest having the transmission looked at. It could be the transmission computer, the shift solenoids could be clogged or could be a problem with the shift position switch.

I strongly suggest, if you do bring it to a mechanic, and they even suggest rebuilding the transmission, to have 3 or more transmission specialists take a look at it and give you estimates. There are mechanics out there that specialize in transmissions only.