96 GTP missing at speed

96 GTP with the 3.4 DOHC motor. Had the timing belt and water pump replaced. Got the car back and the engine will idle and accelerate great, but will start missing around 35-40 mph under a light load. No SES light indication. Have had the motor re-timed 2X, replaced catalytic converter, TPS, MAF, spark plugs and wires, intake gaskets, CPS, and fuel pump. Dealer installed a recorder and we ran it in transmission and engine modes but nothing showed up. The tailpipes look dark to me indicating rich condition. I read post here where a missing or surging condition is present, any solutions found? Help is appreciated.

you may have a vaccum leak or maybe your o2 is bad..