W211 gearbox issues

[Chopper]

I have a W211 2004 E320 Cdi estate with 200k on it. I have owned the car for 3 years now and the problem has been very occasional but now daily.
The car when cold drives fine but when the engine gets up to temp the gearbox nearly always stop changing up unless you red line the engine, if revs drop below 2700rpm then it changes back down. On motorway drives I cant get out of forth unless I do 120mph
I suspect its the transmission plate(A1402701161) and my local specialist has 10 of those on the shelf(it must be very common). I have been quoted £500 for the repair, is this about right?
Its also due a D service, is there any way of checking what items need to be replaced? I was thinking of going to MB/Specialist for the A service and then getting my local garage to change other parts.

 

[Chopper]

Lots of views but no response, I wonder why?

 

[jazzyglobe]

Has the ATF ever been changed along with filter? Have you checked the fluid is at right level? It needs to go onto star Diagnostics to check for error codes. Adaptations can be reset using this too. £500 for a swap of electro plate is a bit much. Should be about half that really.

 

[Chopper]

It had a gearbox oil change 40k ago at MB specialist. The plate from Merc is £199.50 + vat just for the part. I have seen OEM from Germany on ebay for around £110.

 

[b1g1an]

A D service just indicates it'll take four hours, check the service menu for the codes it has derived this from and cross check them with your service book where they are listed.