A/C problem

my automatic A/C unit is going crazy . The temp reading is normal at first start but, maybe after a second start it read 50f. Then when it very hot out and i ldle i can watch the temp go down -16 or today temp of 9 F but it like 90 outside. After sometime it will read the right temp but when it go down so low the A/C will not blow cold air . Please help.