1999 F250 AC works when it wants too... how can I troubleshoot without it costing too much

could have too much freon or condenser too hot = too much head pressure . or condenser plugged with garbage could be getting too hot, an easy to check get the ac working use a hose and cool the condenser and check the ac output, too much head pressure will make the compressor quit working . cooling the condenser will relieve some of the head pressure.

An AC Manifold Gauge set really comes in handy for diagnosing AC concerns. A Tech could tell right away if pressures are high, low, or if the compressor has a valve problem or even a plugged orifice tube. Of course, one would need to know what the readings mean when hooked up. Check the basics first: Air flow in through the radiator, accessory belt(s), cooling system condition, Thermocoupler operation (Engine Cooling Fan Clutch). The only way that you're going to get too much R134a in the system is if you put too much in.