I noticed that when i'm driving around 40 mph, +5 mph and the accelerator is in just the right spot, the engine RPM will oscillate +100-200 RPM. I notice it when i'm trying to speed up just a little bit, because if i press the gas more it will stop, and if i lay off the gas a little bit it will also stop. I also notice it when I go up a gradual hill which requires just a little more gas, and the RPM's will start to oscillate, and if i press a little harder at this point it will usually downshift to get up the hill.

what could this be? and approximately, how much would it cost to fix?