Don't mention Dapol 68s!!!
The problem seems to be made worse when the wagon/coach also has a self-centering coupling. I have no problem with Hornby Mk2f coaches but Bachmann intermodals are a big problem.
After reading these thread I thought I'd do some experimenting.
Roco couplings - didn't even make a 30" radius turn without derailing.
Hunt couplings for Oxford Mk3 (I had some spare) - Perfect,