Hello, Your transmission fluid was leaking from the deteriorated o-rings on the transmission electrical connector. Your transmission was going into "limp mode" because the fluid level was low. Your transmission guy was correct to replace the transmission electrical connector. That is a common issue with the NAG1 transmissions and they will all start leaking eventually. I don't know what you paid but FYI the transmission electrical connector is a $15.00 part and it takes less than 30 minutes to R & R. This is what he replaced . . .

https://www.crossfireforum.org/forum...tml#post832085

FYI - Here's an informational video about replacing the transmission electrical connector . . .

https://www.crossfireforum.org/forum...tml#post832103

Here's a link to explain more transmission repair issues including replacement of the conductor plate (not connector plate)

https://www.crossfireforum.org/forum...tml#post769483

The P0715 code does indicate a faulty speed sensor in the transmission conductor plate. First however, these transmissions are very sensitive to having the correct fluid level. I suspect your continuing issues could simply be due to improper fluid level. Take it back to your guy have him make SURE the fluid level is correct. If it is at the correct level and if you continue to have issues, then you should check again for trouble codes and if you get the P0715 code proceed to get the conductor plate replaced.

Here is a good place to buy the parts . . . NOTE: Use only genuine Mercedes brand parts . . . no aftermarket junk . . .

http://www.ebay.com/itm/Mercedes-Transmission-Conductor-Plate-Connector-Filter-Gasket-722-6-ALL-OE/400746773893?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D33098%26meid%3D5c0c6d04aad9456fb5ae8e4d3401de2b%26pid%3D100011%26rk%3D1%26rkt%3D10%26mehot%3Dag%26sd%3D251359627584
Please come back here and let us know how you resolved your transmission issues . . .