Jerky movement on Athearn Genesis F7 "B" unit

Hi, I have a set of athearn gensis F7 A&B units. The "A" unit works fine, but I find at very slow speeds the "B" unit tends to go normal
speed, then slow, then normal, then slow.
This only happens on the lowest speed settings (I'm using DCC). I don't think its anything decoder related. Also this is very regular, it definitely has a cycle to to.
Any ideas? One of the members at my train club suggested the engine just needed to be "broken in" - but I've given it 20 or 30 laps around the modular layout (probably 120' total around). Could this be the issue, or could it be something else.
P.S. I'm scared of taking apart engines, but I did on this one. Made sure none of the decoder and/or wires were touching the flywheels, etc.
Thanks in advance, Scott
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Scott wrote:

Try taking the decoder out and running the loco on smooth DC. - if it cycles then the problem is in the mechanisim. If it doesn't then the problem is with the decoder.
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Some of these early Genesis engines including the F3's and F7's had a problem with connectivity between the trucks and the motor. I believe it had something to do with either the way the wiring was attached to the trucks or it was the trucks themselves. Do a search on Google and/or yahoo groups and see if you can find out what the fix is.. I know I read about a fix for this issue but just can't remember what it was. PeteC
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