i just had a similar situation, it ended up being a bad injector- i did the test described above, found 1 injector that was the culprit and swapped it with a known working used injector and the problem went away. it was a quick $10 fix for now, til i can get all of them rebuilt with new nozzles and pop tested properly.
once you pull an injector, you will need a new heat shield for each one when re-installing it. so even if you pull yours and have them rebuilt, either get heat shields from the rebuilding shop or order some online so you have them when you put your injectors back in.

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1981 240D ... 265k
1977 240D ... 265k
1977 240D ... 250k