2003 Infiniti M45

WHILE DRIVING EITHER AT 45 MPH OR 70 MPH MY RPM DROPPED FROM 2000 OR SO TO ZERO. THE FIRST TIME IT WAS AT 45 MPH AND THE SPEEDOMETER DID NOT VARY...LAST TIME WSA AT 70 MPH AND THE SPEEDOMETER DECREASED TO APPROX 30 MPH THEN THE CAR SHUT OFF.....IT WOULD NOT START BACK UP RIGHT AWAY BUT AFTER 5 MINS OR SO IT STARTED RIGHT BACK UP WITHOUT MUCH OF A PROBLEM....NO CHECK ENGINE LIGHT DURING THE INCIDENT OR AFTER

PLEASE HELP.....INFINITI DEALERS SAY THAT THEY CAN NOT FIQURE OUT W/O CHECK ENGINE LIGHT BEING ON....