Start the Logitech profiler and find the Force Feedback options, it's easy, it's everything there. Try turning on and off FFB on profiler to see what happens in game. Last, turn it on and off on LFS, Options, Controls. You must watch and try everything. I have a G25 too but I never had that problem, so I can only tell you about see all options, is the way that I use many times to solve a trouble.
Do you have throttle and brake axes combined or separated? They must be separated (Run LFS>OPTIONS>CONTROLS).