I drive a 99 zx2 and just a few dyas ago notcied the car seemed 2 be having trouble doing anything above 50. I drive it on the highways and it will still go 60 but it only stays at 2 rmp or 2000 i guess you would say. It doesnt seem2 be shifting put of 2nd gear. its an autmostix and I feel it shift from 1st to second then i dont hear any other shift the car just stays at 2000 rmp but still goes 50 or 60 it just seems 2 be haivng more trouble then usual. Its a tough problem to describe but if anyone has any ideas please post them. Even if its just a guess let me know..

have you taken a look at your spark plugs could be when you get to certain speed or whatever your spark plugs start misfiring

take a look at them, and if there in sick condition thats probably it

When was the last time you had a Transmission Tune up? Changing the Tranny Filter, Taking the car to a service center and having the Transmission oil Purged? How many miles are on the Trans? What Make/ Model. What happened between the Last 500 miles and Now?! Do you Have any Electrical Problems with the Car now?
I would really start off with a tran Filter( About $10.00 to $25.00, doing it yourself) then take it Aamco Lee Miles, Dealer and get the System Purged( Replacing the old fluid with New) this should be between $89.00 to $ $150.00.