300 feet is a long way for a 30 amp circuit. Did it used to work with the AC or is this a new phenomena?
If it used to work fine and this issue is new, then I would suspect that your compressor may be drawing too much power when it fires up. Could be any number of issues that will require a trip to an AC technician.
If this circuit is new and you've never used it successfully before with the AC, then I would suspect that the run may be too long for the wire size and you are getting a lot of power loss. You can have an electrician check the circuit to make sure it capable of providing the rated power.