With plenty of disk space remaining on a recently-defragmented HDD, and a high-performance processor/graphics card/RAM setup that has had no trouble recording the very same game lag-free until just recently, I am at a loss to resolve this problem. I've tried clean-installing both FRAPS and the video game in question (Overlord), I've enabled and disabled V-sync (it was off originally), and I've attempted recording other games, only to have the same issues. Has anyone encountered a problem like this before? If so, how do I resolve the issue?

Any background processes running that access the hard drive? That could be a smart defrag, Windows indexing service, malware, etc.
Maybe it's a hardware problem. Like loose SATA cable, or a damaged hdd. Check S.M.A.R.T values and run chkdsk.

Upon further testing, enabling v-sync resolved most sustained lag issues.

When analysing my hardware and software, the only major programme accessing the HDD was FRAPS itself and the SATA cable is taught. Nothing abnormal from chkdsk.