Thanks to rs4 and John Webb for those additional thoughts - both definitely worth trying.
Yes, my trusty Hornby Class 60 not only looks and runs beautifully, but it's smooth as silk, never stalls, and is very solid on the track - even at the occasional try-out of hair-raising speed ("just to see if it can").
I like the Bachmann 66s, but if I can only get past this temperament they have....
Granted, relaying the track may help. I've tweaked it a bit, but put off actually ripping it up, for as long as other stock seems to cope.
Fault rectification. My least favourite aspect of the hobby. But, yep, I'll go back to the track and look again.