Is it the original trans? The early cars came with a "Green Dot" C4 (there's a green dot on the shift indicator beside DRIVE) that shifted slightly differently than the later ones. The original shifter doesn't exactly line up with the indicated marks when used with the later C4. If you adjust the shifter rod (it's a turnbuckle type assembly) so that PARK works everytime the you may not be able to reach 1st and can only downshift as far as 2nd when manually shifting. Conversely, if you adjust it to reach 1st, you will have problems reaching park.
In my car I found a happy medium that allows PARK to work, but I still use the handbrake for security. If I slam it I can get to drop into 1st, but not every time. I have to order in the C4 ends for my B&M shifter and fix it for good.