If it were mine under the shade tree, I'd swap in a known good OVP relay and drive it. Clean the intake manifold, drive it. If neither solution proved results, I'd start searching through postings with the keyword "Electronic throttle actuator" and the like. It is possible that problems on the intake side are manifesting as exhaust symptoms.

(This is of course after reading that the harness and egr and associated vacuum connections were all replaced and are currently not leaking.)

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg