RPM fluctuation

[mountainman]

Anyone have a situation where the rpm fluctuates while running along at a steady speed? My 2004 boxster S has about 50 to 100 rpm fluctuation in the rpm when running along at highway speeds. More like 50 when level with little strain on it and closer to 100 when going up a slight grade. It idles perfectly at 850 and has not lost any power and performes just fine in every other way. A couple of thousand miles ago I had a check engine light which the durametric said was a fuel tank vent leak. I cleared it and it has not returned. Has 101,000 miles on it. Recently serviced with plugs, air cleaner, etc. It is a tiptronic. Maybe dirty MAF or low fuel pressure? You would think it would give a check engine light for either of those. Any ideas?