You need at least 20F so there's a problem somewhere. I'd start by installing a manual shutoff on heater hose in case valve is bypassing hot water to heater coil. If that doesn't help start over at square one looking at radiator,screen and other coils. Look BETWEEN coils,shine light to see if light passes through fins. Compressor belt not worn and properly adjusted ? Is it possible someone left shroud off coolant fan or didn't reinstall shroud and fan blade properly following work? Examine ac refrigerant control to make sure bulb hasn't fell of low side line. From here it might be time to call in a pro.