1998 Pathfinder P0300 code

[particlepood]

OK this is my first post to this forum if I have posted to the wrong category please be kind.
I recently bought a one owner 1998 Pathy from a woman who literally poured money into it over the course of a year to fix this DTC code. Some background: the truck had a timing belt replaced 10-15K miles ago and according to her all was fine for about a year. Then the code from hell appeared. She had all the O2 sensors replaced, new ground circuits installed, propane air leak test performed and of coarse a major tune. She has receipts for other ancillary diagnostic testing as recommended according to Nissan's tech bulletins. Since I've had it I've checked the continuity across all the connectors in the harness leading to the various sensors and cleaned them with electronic cleaner. Cleaning turned off the check engine light but when I brought it to get a sticker the SOB wouldn't allow the diagnostic computer to read the ECU and this was after driving it for almost 80 miles over the course of a week. I got pissed brought it up on highway and floored the SOB after about 10 minutes my old nemesis CHECK ENGINE LIGHT glowed with mocking glee. I broke down brought it to a Nissan specialist who installed a known working distributor and again after a few minutes idling the code appeared again.
There must be someone out there that has had a similar experience????
Please share your insights I know I'm not alone!