I just went through the EXACT same problem on my 350SD. I know it is a somewhat different motor, but perhaps it would help for you to read the suggestions that were given to me. The ultimate solution for me was a new glow plug relay and 6 new glow plugs. Since then I haven't had any problems with the glow plugs - and now they heat up in about half the time that the old ones did! Here's the thread:

Another problem with the 603 1991 350SD....

I also run WVO with a GreaseCar system in my 350SD (and my wife's 2001 VW Beetle TDI)

__________________
Autos:
1991 350SD 276,000 miles
2001 VW Beetle TDI 115,000 miles

Horns:
1955 Selmer Mark VI Alto (55,xxx)
1958 Selmer Mark VI Tenor (85,xxx)
1964 Selmer Mark VI Tenor (125,xxx)
1967 King Super-20 Tenor (430,xxx)
2002 Selmer Series III Soprano

For Jazz Saxophone enthusiasts - check out my website...
www.RustyBlevins.com