As Jake mentioned. Every interview you go to for a Diesel Mechanics job is going to include reasons for loss of power. #1 is Fuel Filters #2 Air filters and so on down the line.

You did not say if there was any abnormal smoke while this was happening.

I did not have the issues you were having with the Transmission but you should at least change the Transmission Filter and take a look at what is inside of the Pan. People will disagree with this but if the Fluid looks and smells OK I have occasionally reused it. (Note that if the drain pan has no drain plug I install one; there is generic kits to do that.)

Note that there is a maximum rpm setting at which the Governor starts cutting off the fuel to the fuel injection pump. I don't know what it is on yours but on mine it is 4500-5000rpms.
With out knowing the specific specs on your year and model I cannot say that yours is set too low but you could find out what it is supposed to be.

If something is keeping the crankcase pressure from venting out and your fuel injection pump has a vacuum shutoff the pressure can push that vacuum shutoff in the shutoff direction.

Your Fuel Supply/Lift Pump could be failing. There is a rebuild kit for that an instructions in the Repair Links section.

If you have a turbo check the boost pressure. Check to be sure that the boost pressure line is clear.

On my year and model there is something called an Over Pressure Relief Valve. That cuts off the Boost pressure to the Fuel Injection pump which intern turns your engine into a non-turboed engine but with a restriction in the air inlet.

Then there is that smog thing on called the Trap-oxidizer that like a Catalytic Converter deals with some of the emissions but in the case of Diesel they get plugged. On you year and model it might be called something else.

I don't know enough about the transmissions to give much advice on them.

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84 300D, 82 Volvo 244Gl Diesel