If you happen to have an Intel P4 and the temperatures were such that it's thermal throttling mechanism was kicking in then the answer would be yes. But (assuming you have an Intel P4) more than likely a case fan would not change a problem causing the throlling back to kick in. Generally more direct cooling of the process would be needed (ie the heatsink and/or heatsink fan).
If your processor is an Amd, then the answer is no. Absent a form of thermal throttling, then heat issues surface in a kind of "all or nothing" way. In other words, your system either runs normally, or you start experiencing lockups, freezes or reboots. Absent thermal throttling the point between when the processor starts slowing down (due to high error rates) and the system freezes/locks/reboots is so fast that you wouldn't even notice it.
Please remember to post back whether your problem is resolved or
not, so that others may gain from the knowledge.