I'd bet it is the "electrics kit" on the conductor plate inside the tranny which goes haywire on these. Not too expensive...the part is about $200-$250 and labor to install is just a couple of hours. If it turns out to be that you should insist they give you new fluid and filter and drain the torque convertor while they are in there which is now recommended maintenance on them anyway. That will save you the labor of having that job done spearately.

__________________
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz