300SDL that hates cold weather...

[Z28SB350]

I must say that I have been very impressed over the years reading and reviewing the many posts of priceless information on this forum. I typically search and search until I find a solution to whatever seems to be ailing my '87 300 SDL, but I seem to be having some difficulties this time around.

The engine has always been rough whenever it is first started. Doesn't matter if it's 25 deg outside or 80. It's never been a problem though, and it's always smoothed out after no more than 30 seconds in either case. Then it purrs like a kitten.

For the last few weeks, I've had an issue when it's cold outside and I'm trying to get up to speed. In any gear, it appears that the engine has a hard time getting over 2500 rpm. It stumbles and almost feels as if someone is randomly pumping the brakes on me! After reading through several posts, I decided that I would try removing the ALDA unit off the IP. I was astounded by how quickly the car accelerated off idle, but it still didn't fix my problem.

This morning I went out and fired up the car (20 degree morning) and for the first time ever it died about 10-15 seconds after starting. I immediately initiated a second glow cycle and fired it back up. When I finally took off (let it run for a few minutes) it had a great deal of trouble getting going. It was stumbling all over the place and I noticed a good mount of bluish smoke coming out the back. This was the first time I've noticed this. After only a mile or two, and never getting over 45 mph, I finally turned back to the house and got my backup truck to get me to work (12mpg, by the way). Also unusual this time was the fact that the car started stumbling around 2000 rpm, lower than before.

Just for additional info, the car does use a bit of coolant. It's got a model 14 head (yes i know this is bad) but I had an SDL before this one that made it to 400k miles before this became a problem. Also, the car runs much better after it gets all warmed up, but it still stumbles before it gets ready to change gears. I assumed the tranny might be trying to give up the ghost, as I am expecting this any time- but it doesn't make sense to only have this issue around a certain rpm range. Another bit of confusion is that when the car is in park, it will easily rev up beyond the 2000 rpm, so it's obviously only an issue under load. Plus when I'm reving the engine in idle I can tell that the turbo appears to be functioning.

Anyone dealt with this before? Could it be something small like a fuel delivery issue, clogging filter, faulty injector?

Additional Info:

I've had the car for about 2.5 years
326,000 miles
Transmission was flushed at 287k miles.
Turbo was replaced (under oxidation catalyst recall) at 287k miles.
Always run Diesel Kleen through it after every 3k mile oil change (use Mobil Delvac)
Engine has the newer style vaccum pump which is working fine according to my mity vac. I went through the steps of checking to see if I had a transmission shifting issue due to vacuum, but everything appears to check out ok.
K&N air filter