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Athearn's "SW-7" model ran great, even the old ones with the silver motors and silver flywheels with proper tune up, cleaning and lube. There have been countless articles both in print and online on how to accomplish this. The only possible exception is a bad motor, which you, then, of course, replace.
Mike
Broadway Limited also makes a SW7 but no Southern just undec: http://www.broadway-limited.com/search.aspx?find=sw7
They are always showing up on eBay, Yahoo Group selling forums & internet MR selling sites - usually cheaper.