Air conditioning blows warm air, cold only sometimes

A/C that cools and then warms up, and works better at highway speed, can come from a few different causes: a low refrigerant charge from a small leak, the compressor clutch disengaging (sometimes because the clutch gap is worn), a condenser fan that is not running at low speed, or a blocked condenser in front of the radiator.

We would avoid simply adding refrigerant from a can. If there is a leak, it comes back, and an overcharged system can cool even worse.

A proper A/C diagnosis includes checking high and low side pressures, confirming the condenser fan runs when the A/C is on, inspecting the compressor clutch, and doing a leak test. That points to the actual cause so you only repair what is needed.