Another idea to try....
Sorry to read that you're having troubles! You might also think about connecting a high impedance voltmeter to your O2 sensor and monitoring the voltage while you are driving - you would surely see at least the result of what's happening as far as a rich/lean condition goes. It sounds to me like the engine must occasionally starve for fuel, and the function of operating the full throttle switch gives it enough fuel to run again.
By the way, I'm thinking of removing the dual rochester side drafts on my '77 Jag XJ6 and installing TBI from a 4.3L engine in their place, just to have a better running vehicle. (The Jag really starts hard in the winter with no real enrichment system)
Good luck on the problem!
'82 300D/4.3L V6