Power loss in 93 300D

[spwilliams]

I have recently started to experience a power loss in my 93 300D with 312K miles.

Here are the symptoms. It started a week or so ago and manifests itself when going up hills. It only seems to happen when I am in 3rd gear, IOW I can accelerate normally (slowly) from 0-20 or so, and then the car will not seem to shift into 3rd unless I let off the accelerator, the it shifts hard and has no power. It seems to be getting worse. Today I could barely get the car over 30 when entering the highway on ramp up an incline. It seems to bog down at around 3K RPM.

Bad gas? - I am a newbie deisel owner, first winter and could this be the fuel? I'm skeptical...

Clogged ALDA? I know that this can cause power loss from this forum, but would it happen relatively suddenly?

Compression loss? Wouldn't this be an issue when starting out as well?

I'm bringing the car to the mecahnic later this week, but can anyone arm me with some facts and questions to ask? Things to test to eliminate any or all of these? I don't know the mechanic THAT well and its will only be the 2nd time I've brought the car to him.

Thanks in advance for any insight.