[chittychittybangbang]

You probably saw this article http://www.myturbodiesel.com/1000q_how_to/multi/limpmodetdi.htm in the other post. If you can go past 2800-3000 revs on local roads without this problem it's something relate to the greater load on the highway.

My first suggestion is to scan for codes. Maybe fuel restriction, maybe cracked vacuum line.

 

[chittychittybangbang]

Most likely vaccum line, sticking vanes, clogging intake, bad VNT actuator. You need to do more testing to narrow the causes to one of these, see the article linked above for detailed steps. For best results, you need basic tools like a vacuum pump and VCDS or another computer tool to let you read the sensors.

 

[chittychittybangbang]

Does going uphill trigger it in the lower gears too? With positive boost deviation, it should. While North America didn't get that engine it shouldn't be too different, so it may be pointing towards sticking vanes. Log boost actual vs. boost requested with a VCDS or similar too. You'll probably see evidence of the positve boost deviation. If you've already done a boost leak test, you said you replace the vac line and actuator is fine and moves through the correct range and holds vaccum, and the N75 solenoid is working properly, it would suggest sticking vanes.