'01 Accent w/ RPM Fluctuation when Driving

[deehuff84]

Have taken car to two places... two diagnostic tests, 100 miles driven on a portable scanner with no codes, and a fuel system cleaning but it still has the same problem and the places tell me there is nothing wrong with the car... when Im driving down the road the RPMs will fluctuate up about 300 to 1000 and then drop back down while I am driving at a consistant speed, but doesnt matter if its 70mph or 40mph... Wondering if it has something to do with the Flexpipe or the O2 sensor I had replaced because it started having the problem right after those two things were done. Can anyone help pinpoint the problem?