Just to clarify, you cleaned the actual OPR switch, or the banjo bolt and plastic as in the DG tutorial? I'm also hearing carb cleaner from you and brake cleaner from DG, do you think it matters?

I'm having a similar problem with my 85 300DT. I noticed, however, that it is mostly slow to accelerate from idle, and has some pep if it's at 30 or more mph, so this might not work for me. Going to give it a shot anyway.
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Cheers!