shifter mechanism

I think I have written this before I bought a screw (2 bolt) shifter from Brewers as I did not know that I had a bayonet type orinal (69) barracuda and they put it in without the bolts and is held in withn that shim and had no problems but I nbelieve my shifter mechanism now is not workingn well is itm possible the shifter is not in well because sometimes I have problems with 3rd, only 3 third and at higher rpms..