Error code 49409? any ideas..

[SamBaxter]

i own a 2007 gti , 2.0L FSI with 60000km, it has a few modifications but its stock ecu at the moment, it has a dsg gearbox, and has changed issues with gear changes.
at low revs and high load the car jerks forward, loses power , then regains power and does this again for the next gear change,
the issue is more pronounced if i alter my stance on the accelerator, and at times the whole car has nearly cut out because of the sudden erratic gear change.

the code reads ;
SCAN ENGINE AND TRANSMISSION CONTROL UNITS
***FAULT CODE NO. 49409/NO COMMUNICATIONS WITH TRANSMISSION
CONTROL UNIT/008 IMPLAUSIBLE SIGNAL
CLEAR CODE

so any ideas what causes this error ,
car is booked in to vw for nxt tuesday so ill let you know if no one posts it here.

 

[SamBaxter]

i got the car back from vw today, i was told it was a can bus issue and that by re grounding a few wires it had the problem solved... they said they drove it for 120km and couldn't recreate any fault, so i got my key and went out to the car , sat in and drove off , literally a minute into the drive and it did it again?

this problem seems to be impossible to find? does anyone have any knowledge of this ? its no longer a mechanical problem but an electrical one so i know I'm in the wrong section here but bare with me,
could it be a fuse , or the battery? i have an aftermarket pioneer head unit and subwoofer?
what is the can bus system? and could it be that the ecu is screwed?
I've changed all the physical parts that could be causing it , so whats the electrical stuff that could be doing it?

 

[SamBaxter]

brought the car to vw there, they did a full scan and found no fault codes at all

so my only options now are to check for things myself, i think it could be a bad fuse, battery or wiring , any one any ideas on how to spot a dodgy fuse? and how to check the battery ?, advice appreciated