I agree with John, and have experienced the same symptoms you have described.
A word of caution. Sometimes the regulators just quit, and smoke ceases for good.Other times they can go out in a blaze of glory.
This will produce a ridiculous amount of smoke for a brief period and creating heat that could risk damaging a plastic shell.
I've only experienced this on a die cast steamer so there was no damage to the loco. Some have even reported seeing the resistor glowing in the stack.
It might be advisable to shut off the smoke with the switch on the loco until you replace the regulator.
