I suspect the meters are not reading properly with the open frame motors (they may not be ready correctly, period). Everything you are describing indicates you are pulling current well over the 6 amps displayed. Since the train behaves if you back down the voltage to 16 I don't think this is a noise issue either. I'd recommend getting a hold of a decent meter and checking the actual draw or try running that train configuration on a straight PH 180.
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