It might be that the original DAT which you modified was Interlagos'. (s1ct03.dat) Interlagos has by far the bumpiest surface of all the original tracks and this behaviour is set in the magic data. You need CMagic 1.02 to extract this magic data from the GP4.exe, modify it and insert it directly into your DAT file from which GPxPatch could read it.
At the moment I don't know where you can get CMagic, but I'm sure someone will soon give you directions.
P. S.: Monza is also very bumpy by default as I recall. The above notion will apply to that one as well, if you happen to use the Monza DAT.