I have been to several other auto forums on the engine surging at 2000rpm
to 2500rpm at highway speeds(above 50mph) and they all suggest one of two
things.
1. new spark plugs and wires
2. Clean or replace fuel injectors. Good reference site:

http://www.automotiveforums.com/vbul...&highlight=rpm

The first of the two is the cheapest route to go.(~$40.00)

The second seems to be very expensive if you replace
them.($115.00/injector) If you have them cleaned it seems to be a little
cheaper.(~$85.00 to $90.00) If you would like to know the steps to
cleaning them this web address might help:

http://web.ask.com/web?qsrc=6&q=Clea...r+Yourself&o=0

I hope that this helps. Good luck!