MH,
Aside from the normal "center the shifter lever L-R and Fore and Aft"......
Check the shift rod where it passes thru the firewall. If you have a rubber boot at that point, if the rod is too low or high (trunnion adjustment!), the shift rod can contact the pass-thru-tube, or wad up the rubber boot at that point and cause issues like you suggest.
I overhauled my ZF just because I "fixed" the trunnion adjustment height......which then caused 2cnd and 4th gears to pop out..... Nothing like fixing something that isn't broken.......!!!!!
The ZF is a pretty stout unit with good engineering! It's only when funky engineers mess with things that it "goes bad..."!
Cheers!