Car is hunting on the highway @ around 2500rpm.

[Sal]

2004 Aveo LS Automatic. Ok I am not sure what the actual term is but in the electrical world (generators in specific) when a generator's engine is hunting, the rpms tend to drop the rise then drop then rise...etc but its slightly rapid, say 20 time a minute. my car will start to do this at highway speed when im in the 22-2800 rpm. And just for explanation purposes I have not noticed it at idle while stopped. what throws me off is that Im not getting any check engine lights either. if it helps I put lucas fuel system cleaner in my gas tank ever 3000 miles. Anyone have any explination as to what it could be, or some advise as to what to check first?