Hi Mike,
Sorry, I didn't spot it was a non DCC topic but carrying on from what DCB said did you buy any of the units second-hand in which case there is a slight chance they may have DCC decoders fitted but running in DC compatibility mode.
You said it was not controller specific but were all the controllers tried of the feedback type?.
There is just a chance that feedback controllers may struggle with multiple units.
Also I find it very difficult to judge slight changes in speed of a model train - maybe coupling together units is showing variations in electrical pickup.
Do the units have Kadee buckeye couplers at the cab ends - that may reduce the effect of surging.
Regards
Nik (EMU and DEMU mad)