From my experience, you're right. It sounds like there isn't enough venting from the attic to the outside. What you're experiencing is, basically, back-pressure. But before you go cutting any more holes, you need to do some research first.
Try to note the make, model, and serial number of the fan and search around to see what the proper venting should be. Perhaps the previous owners can give you some paperwork, or at least the M, M, and S#, if you can't seem to find it on the fan. Proper venting is essential to getting adequate cooling in the house. You should also measure the lenght, width, and heighth of the attic as well as the current venting. You'll need this to see what the proper venting should be. And as an aside, a more desireable setup is to have a combination of gable, soffit, and ridge vents to keep the attic and house at the proper temperatures.