My ac works and blows well at times...at times the ac will just blow hot air. I will turn off and turn back on a day or so later and then the ac works fine. 1999 chevrolet silverado 1500. any help?

The air temperature control blend door is sticking in the heater or neutral position.

Duplicate question.

Check to see if the radiator fan is coming on & the AC fan comes on when the AC is turned on. AC Freon may be low too.

To clarify - - if the truck has rear and heat, you have two heater cores and two evaporator cores. The evap cores carry the freon.
If the leak is in the evap core in the front, the price is probably right.

You likely have a restriction in the heater coolant hose circuit - perhaps a coolant control valve not fully opening, or a kinked hose.
Insufficient coolant volume is flowing to keep the heater core hot.
Try back flushing the heater core system with a garden hose to check flow.

is the compressor turning? If not, replace the AC clutch relay. $11 at rockauto.com

possibl ac compressor clutch problem,cooling fan at rad not working so ac goes into high press mode and turns off(so it doesn,t blow system apart,built in safety)not enough r134 in system(low charge)

You have a vent control actuator motor stuck,or gone out.

this is because you have a bad switch which has gone bad and you need to replace it or either try to bypass it through another circuit.

Sounds like a possible broken/ripped/disconnected air line (hose) to the compartment.