We just replaced the fan motor and the starter capacitor and it starts running just fine and kicks off and then kicks right back on and about 30 seconds later it kicks of again and kicks right back on. What is causing this?

Possibly a defective contactor.
Pull disconnect to kill power.
Remove cover.
Check all connections, both high voltage and low voltages terminals for good connections.

If you are comfortable working around LIVE HIGH VOLTAGE, replace disconnect and set thermostat to call for cooling. Observe but do not touch contactor. Contactor should make and hold a connection. If it makes and breaks, you should be able to see and hear it doing so. If contactor makes and breaks and the blower motor in the air handler is running contentiously, the contactor is likely defective.

Pull disconnect and check low voltage terminals. Should have 24 volts or more across terminals. Remove low voltage wire and do a continuity check between the low voltage terminals. Inspect high voltage contact points for burned points. Contactor may be single or double pole. If you have 24 volts on low voltage and blower motor in air handler is running the contactor is most likely defective and failing intermitantly.

If you do not have 24 volts at low voltage terminals you may have faulty control board or relay in air handler, faulty thermostat or defective low voltage wiring between air handler and condensing unit.

Could also be a thermal cutout switch.