This sounds real weird...
I'm sorry if I cover some things that have been done already, but just working through this logically here.
-Check the Gpx executable on the GPxPatch tab
-Make sure there isn't a season set in the GPxSet tab
These seem to me like the only things that would change the liveries and so on from GP3.exe to GpxPatch. All else really should be the same.
I use all the things you do except my version of GPxPatch is 3.91, so all I can see the problem being is GPxPatch itself, and everything else is pretty cool. If all else fails, I would try giving that a go.
_________________________________________________
Williams' Wonders: Jones, K.Rosberg, Piquet, Mansell, Prost, Senna, Hill, Villeneuve, .....?