What could cause my split unit to work properly during daytime, but not at night? The compressor or fan won't run at night when the temp is cooler outside. The unit blows air when "told" to by the thermostat, just not cold air. Everything works perfect the next day. Been going on for about two weeks now. Thanks for any tips.

Could be your pressure switches telling the system not to run. Could be a freon issue.
I suggets having a tech come look at it. Make very clear to the situation of day vs night. Otherwise they won't know what to look for.