Hmmm - after checking for significant air leaks in the intake system, including splits in the large black air hose, my guess is unfortunately, you have a bad catalytic converter.
The internals of the cat have quite possibly broken up, and occasionally plug the outlet pipe. After shutting it off, pieces fall back down and the blockage is gone.
They're about $125 from FCP Groton, new, or possibly you can get a shop or junkyard to sell you a more or less complete system- though officially, junkyards are not supposed to sell catalytic converters.
Good luck!
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Rob Bareiss, New London CT ::: '87 244DL/M47- 234K, '82 245T/M46-182K, '89 244DL/AW70- 212K Not too distant past: 86 244DL 215K, 87 244DL 239K, 88 744GLE 233K, 88 244GL 147K, 91 244 183K
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