I'm looking to cool a 36x28x9 garage with a single 18k btu unit. Good enough?

As seems to be the case in several other posts, our outside AC compressor isn't working as I type this ...but it may turn on in the next few minutes or tomorrow ...or not. I've searched through many posts with similar systems, but haven't been able to narrow down the fault for certain and would appreciate your guidance. My unit is a Lennox AC13 outdoor unit and has a C33/CX44 Series coil inside. Symptoms: Outside compressor & fan sometimes turn on correctly, other times wont turn on at all, and other times the contactor will connect, then disconnect, then connect again, clicking several times noisily before finally falling silent. When I set the thermostat temp low enough to start the unit, there is an audible click inside, but often neither the outside unit nor the inside circulation fan starts. I left the thermostat on after it failed to run all afternoon and interestingly, it has just started up again (9pm). Very frustrating! Checks thus far (while AC not working): * There are no tripped circuit breakers inside or out (my outside unit has a breaker) * Replaced capacitor outside (although my meter indicated that the original cap was within tolerance, it was too inexpensive not to try.) * Tested voltages: There is no 24VAC at thermostat, outside unit entry point, or contactor. * Manually closed the contactor: both fan and compressor start * High and Low pressure switches: Neither needed to be reset * Switched to Heat: No ignition and no circulation fan. * Swapped the thermostat with a 2nd, functioning unit: No change. * Inside unit is upstairs: checked that the condensate float switch and the front panel kill switch were both working correctly (using with my meter). I believe that I've eliminated the breakers, contactor, capacitor, thermostat, and hi/low pressure switches as the source of the problem. I'm tempted to replace the control unit, but wanted your advice first. Thanks in advance!