This is harder. Measure the voltage from the circuit breaker buss to the breaker output. For both buss's it should be near zero. If it is good then measure the garage panel. If you find a fault it may be the interconnecting cable. Hopefully the cable is good with no splices. Somewhere you have a problem. If you are very lucky it will be in the panels not the buried cable. A digital meter samples the voltage.I The two phases ( not really ) must be about the same. If one side is erratic then that is the problem. Unlike an analog meter a digital one periodically samples the voltage . That may disguise. the problem.