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.
- new spark plugs and wires
- Clean or replace fuel injectors. Good reference site:
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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:
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I hope that this helps. Good luck!