Abstract: Improved methods and apparatus for testing and diagnosing defects in data processing networks and circuits. The presence of a time-dependent error in a network is determined by testing the network at normal and reduced clock frequencies. If a time-dependent error is found to be present, the clock period during which the error occurs is then determined by repeating the testing of the network at normal clock frequency a predetermined number of times with a different selected clock period being extended for each such repeated test operation. The amount of the clock period extension is chosen to be sufficiently long to prevent occurrence of a time-dependent error if it should occur during that clock period. The error clock period is then determined based on a determination of the particular repeated test operation for which the time-dependent error did not occur.