Hey all - this is relating to my ongoing saga of repairing my girlfriend's 76 300D (there's another post somewhere...).

I just got the car running again following an overheat / non-start sequence of problems. The car starts and runs now, but if I let off the throttle completely, it dies. I'm willing to entertain the thought that it could be the idle control having 'wandered' and needing to be reset, but there's a second problem as well: the car now stalls when I put it in gear. If I shift into gear at [mid-range] rpm, the car will surge forward a couple feet and then die. If I shift into gear at idle, it just bogs and dies.

Any thoughts? The overheat melted a bunch of Viton fuel lines (leading to the non-start, in part), so I'm wondering if I fried a vacuum line or two. The only other time I've seen similar symptoms was when the IP went out on my newer 300D engine ('83)...

When I first read the post I was thinking that there was a previous Engine/coolant/stuck Thermostat type over heating.
But when I read about Viton hoses (makes me think alternate fuel) being melted I am thinking there was some sort of Fuel Hearter added that malfunctioned or there was a Fire somewhere in the Car where the hoses got melted.
How about some clarification!
Does the car have an Automatic or Manual Transmission?
If the hoses Melted maybe something moved up and plugged up the filters.

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84 300D, 82 Volvo 244Gl Diesel

Good point - sorry for the unclear post.

Overheat was because of either a stuck thermostat or busted water pump.
My girlfriend was driving the car when it happened, and the temp wasn't registering as super-hot on the gauge (in fact, the radiator was cold...)

The fuel return lines between the injectors had been swapped to Viton or similar, back when we were considering a WVO conversion (we've since ditched the idea).

The car has an automatic transmission.

I went ahead and replaced the fuel filter as part of getting the car running again. It runs okay with a bit of throttle all the way through WOT, just doesn't stay running at idle or with the transmission in gear.

I have never been told about or experienced an Engine that overheated enough to ruin any of the rubber hoses! So it must have been extremely hot to do so.

I am thinking that if Viton Injector return lines got hot enough to melt the there is a good chance the Injectors suffered some damage. May be even the little piltle tips got burned.
You might want to pull 1 or 2 Injector and have a look at the tips for discoloration or burned Pintle tips. If the steel is blue from the heat it will be softened and need to be replaced. (If you pull any Injectors take a peak at the Ball Pins in the Pre-combustion chambers.)

However, that type of heat could also damage/soften the piston rings and glaze the Cylinder walls (when it is running is there colored smoke).

You might also have a canidate for a Compression Check.

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84 300D, 82 Volvo 244Gl Diesel

The lines are not rubber - they're some sort of synthetic, closer to PVC tubing. The shop I bought the lines from said they are not intended for use in an engine bay; they're apparently meant for diesel lawnmowers? (Yes, I'm annoyed at the shop, but that was their 'diesel safe' line)

I'm going to do the compression check and injector check tomorrow.

Are you getting fuel like you should to the engine. I might take a look at the tank strainer and make sure it is free of any sludge. Also make sure the round channel that is just below the actual fuel tank where your fuel goes into the fuel line is clear. I have seen junk get in that channel and clog up the entrance for the fuel to enter the line. It may run fine for a while and then the block will happen again and the car will die. JUST AN IDEA.