why when driving at 60 to 70 does engine start to loose speed and stall

Could be any number of things mass air flow sensor, low fuel pressure, restricted catalyst . You need to bring to a shop that has expertise with your BMW like Jensen's Service

Had 2 diagnosed 1 at BMW dealer. O2 sensors changed, thermostat, idle control valve, new air filter with filter pad, mass air flow sensor . Still all warning lights come on and runs rough, gas mileage drops off bad, wants to stall. Sometimes after 20 miles other after 2 miles. 4 months of trouble. Please help

Could be a plugged up fuel filter or water I your gas tank. I had a 2000 323i that would do the same thing when you went around a sharp couner. It had water in the gas tank. Pulled the fuel filter and it was full of water and plugged up. Bought some of that bad cheap gas somewhere.