What's your FFB setting like (if it's GT5, from 1 to 10). Also take note prior to start of racing/driving in GT5, the setting regarding Traction Control etc.
Having owned the G25 (essentially the G27 with less buttons on wheel and regular non-helical FFB gear), I find after 30 minutes of continuous use, there's a distinct drop in FFB quality. There's also the fact that after 10 minutes or so, your arm/shoulder muscles would've adjusted to what seems like a lot of force at the beginning. If this is the problem, you might want to start with lower FFB setting and turn it up the longer you drive.
A couple more things- make sure the G25 is set to controller #1 on the PS3- you do that by either not using your gamepad and if you must, press and hold the PS button on the gamepad and in the controller setting, pick controller #2 for the gamepad. I find when I have the G25 connected to other than controller #1 setting, the FFB feels slightly different.
And finally- there were various times when I found the FFB feels distinctly weaker than what I'd felt before (in GT5) and by simply disconnecting and reconnecting the G25 to the PS3 (the USB cable/connector) I would gain the old FFB again. I think it's a simple resetting method that seems to fix the issue.
If all these fails- it's time for a stronger wheel like the T500RS
