On my Car for a while I had no shift into First Gear at all unless I reved the Engine. It would also not go into First Gear. The above started during the 1st couple of days of some Cold Weather.
It appeared to be a B2 Piston Issue. I did not have time to fool with it so the Car was driven starting in 2nd Gear; shifted there Manually.
When I had time to do so I changed the Trans Filter and now everthing I now have a 1st Grear and everthing seems as it was. The Weather is also slightly warmer also.

It may seem unlikely to start troubleshooting with a Fluid and Filter change because the chance that it will help appears small. But, it rules out a possible source of a problem.

Some of the stuff I read indicates it could be a Transmission Modulator Problem. But, again as the others said you still have to make sure your other adjustments and that the lines are not restricted in order before fooling with it.
You might take a look at the Cap on the Modulator. Perhaps there is a Crack in it the Cap that causes problems when cold.
An article on adjustments:
http://www.continentalimports.com/ser_ic20242.html

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84 300D, 82 Volvo 244Gl Diesel