my 2.8 olds cuts off when warmed up

My 1988 olds Cutlass Supreme with 2.8 cuts off just before the cooling fan swithces on and will NOt restart until the ngine is DEAD cold again,, Ive replaced coil packs, coil pack board , ECM, new updated prom, its getting fuel, spark and air, the mechanic has had it for a month, no progress. He keeps telling me its NOT the crank sensor, its NOT the engine coolant temp sensor.. this has got us perplexed. its a sweet car otherwise. clean as a pin, and sharp, all power options and the FE3 suspension,, Why cant this guy with his computer  and his 25 + years as a GM mechanic, not figure this out?? Help please..btw its running full Synthetic oil in it too