then probabely the slipstream effect is enabled in gp4tweaker if you disable it or adjust the effect maybe it goes better !!
you go into the folder tweaker and open the gp4tweak.ini file:
SlipStream = 1 ; set to 1 to enable slipstream effect
SlipPlayer = 1 ; set to 1 to enable slipstream affect to player car only
there you enable or disable it !!!
to adjust the value of slipstream you go into the folder gp4tweak_data and there you find the slipstream.ini file were you can adjust for each track and driver the slipstream value !!!