Car Rocking While Driving

I have owned my Cobalt for about a year now and I
have noticed this issue pretty early on since I
purchased it. When I am driving my car at low
speeds, usually 25-45 mph, I will notice my rpm
needle bounce up and down ever so slightly and
my car will rock forward and back ever so slightly
too. This rocking only lasts a few seconds or so
and often occurs several times throughout the
drive. It isn't like a jerking motion either. Like I said,
it's a smooth rocking. It is also not always
consistent. It doesnt happen every time I drive it,
but it does the majority of the time. The bouncing
will also be more or less noticable with the rpm
needle jumping anywhere from 100-300 pms
depending on the drive with the noticability of the
rocking corresponding the the amount. I keep on
on maintainance, but am a little short on money
since I am a college student. I have had several
mechanics test drive it and have no answers, but I
dont really have the funds to do any thorough
inspections on the car. I have been told several
possibilities ranging from possible transmission
issues (hopefully not), to a worn out fuel pump, low
fuel pressure etc. I myself have little car knowledge
and would like to know what could possible be the
issue at least to ease my mind a little. I have
owned the Cobalt for about a year now with no
changes to the issue, so hopefully it stays that way
until I have the funds to figure out the real issue