Simon,
Ive not had the problems you have experienced,but i had 3 Bachmann Class 66's i had to get rid of.
I still have 4,but the 3 in question didn't like going through the points, either through the straights, or the curves.I have no small points on my layout,as there are alot of loco's besides 66's that don't like those as they are too tight,so it wasn't the points. I found that the older Bachmann 66's, for example 66 135, didn't have pick up's on all wheels, therefore making them vulnerable traversing insulfrog points. Also,if you have attached the front bottom plate,this too sometimes interferes with the bogies movement when taking on curves. I had this problem with 66 411. Also brake pipes etc can foul when turning if not attached correctly. After all said and done though,i was forced to get rid on E Bay, which cheesed me off somewhat as one was a sound version, but just kept getting stuck through certain points and causing a short circuit which switched off the sound. Very annoying !
I have a DCC modern image layout,so 66 omission isn't quite right, but i cannot put up with loco's that won't run smoothly either through point work or any other area.
Just for the record,my listing of 66's is as follows:-
66 135, No good, hated traversing the pointwork
66 522, The same
66 022, The same
66 405, Still got this one but doesn't like SOME points
66 407, This one runs ok
66 411, The same
66 412, The same.
Everything else i have, ie, 37's,57's, 60's etc are excellent runners.
Hope this helps
John