Methods and apparatus are disclosed for conditional acquisition of potential measurements in integrated circuits, with the aid of electron-beam probes. The conditional acquisition enables display of waveform images which permit diagnosis of the causes and/or origins of failure in circuits which fail intermittently. Data is acquired in the normal manner on each pass through the test pattern. At the end of each test pattern execution a pass/fail signal from the tester exercising the circuit is used to reject or accept the acquired data. In this fashion, it is possible to accumulate only that data which carries information about the failure of interest and to reject data which does not. Over several test pattern repetitions it is possible to display only that data which shows the failure. Engineers are thus able to efficiently diagnose intermittent failures without the need to change device operating parameters. In one form of the invention, (a) initially a first buffer is defined as a "good-data" buffer and a second buffer is defined as a "temporary-data" buffer; (b) the circuit is probed during application of a test vector pattern to acquire data; (c) the acquired data is summed with stored data from the "good-data" buffer, and the sum is stored in the "temporary-data" buffer; (d) a determination is made whether circuit has failed to operate as expected in response to the pattern and, if the circuit has failed to operate as expected, the second buffer is redefined as a "good-data" buffer and the first buffer is re-defined as a "temporary-data" buffer; and (e) steps (b)-(d) are repeated, such that the data stored in the "good-data" buffer represents failing operation of the circuit.