I had this happen after about 15 minutes of play. Helped with this problem launching the game as an administrator, after that no problems. And if you have just a temporary drop in FPS, then try to turn off the puddles. (Google translator)

Fixed this! Run the game in Win 98 Compatibility, Disable Puddles (in-game or config.ini). Also, you can turn your FPS rate to 60(maxes at 31 but it may push the limit a bit to set 60.) Hope this helps!