I have several recommendations. 1st, your 300D turbo should be faster than a striped a$$ ape on the highway. Mine will run to 120mph and maintain it effortlessly. You have some problems to take care of.

Some other maintence items worth checking out:

Valve adjustment (I go over the valves every 20,000 miles)
This could cause a loss of power and in severe cases burnt valves.

Regular transmission fluid&filter changes.
This can help prolong life by keeping clutch material out and keeping everything clean and fresh. If this hasnt been done regularly, be VERY careful. Transmission seals tend to "adapt" to the chemisty of the fluid and when new fluid is introduced the seals will just fall apart. If your transmission hasnt had regular services, this isnt the time to start.


Now on to your main problem. maintaining speed. It is possible you have a turbo problem. The wastegate may be either stuck open or weak. You also need to check out the boost fuel adjustment aneroid commonly referred to as the alda. this could be clogged from years of EGR soot and CCV oil vapor. Youll need a competent shop to perform more advanced checks.

Where are you located? Let me know, I know some guys who are very good all over the country.

__________________
Matt
2003 Jetta Wagon TDI 145,000MI
1999.5 Jetta TDI 287,000MI
2001 Suzuki SV650S 26,000MI
2008 Yamaha Vino 125 11,000MI
http://www.crazymatt.org