Bike won't go faster than 30 mph

[sphinx_57]

Ok I got a 06 gz250 with 1300 miles. I got it with about a thousand and have put a few hundred miles on it in the last week or so. When I got the bike it had sat for about a year, mabee started twice. Took a little time to get it started the first time, but seemed to run pretty good. I have burned through all the old gas and have fresh gas in the tank and had added some cleaner to the last tank. So today I started to head home on the highway got to about 65mph and started to get a bad surging. I got off the road and found that much more than a few thousand rpms and the bike would surge badly and there was no power in the throttle.
I checked for spark which was good, the spark plug looked pretty new, I drained the bottom of the air box, checked the air filter, and still has the same problem I don't dare try to get past third gear at such a low rpm. It idles fine, starts up easy, and doesn't seem to have any strange noises.
Things I noticed the last week driving it. Power seems decent for me since its my fist bike, but it has always had a hard time getting up to 65 in 5th gear. I did go on a small ride with a buddy the day before and 60mph was tough to maintain. I figuared it was alittle windy. Also there is some rainbow color to the exaust pipes were it exits the heads. Any ideas? :skull: