1998 BMW Z3 Car keeps cutting out while driving

This high mileage BMW has been well maintained, It is used to commute between Utah and California. Last week it started to cut out while driving as if someone turned off a switch. It is not a ground problem, the lights stay on but the engine stops running. I have replaced the DME relays, cam balance sensor, cleaned off all connections to relays and computer. The problem is progressivly getting worse. I could drive it for 1/2 hour at first before it would start cutting out, now it is 5 minutes. It runs very smooth but without warning cuts out as if a switch has been turned and then abruptly starts up again. Please help