On July 28, 2002, about 1349 eastern daylight time, a Fairchild Dornier DO-328-300, N409FJ, operated by Atlantic Coast Airlines (ACA) d/b/a Delta Connection flight 6107, was not damaged after the crew reported a loss of flight display information shortly after departure from LaGuardia International Airport (LGA), New York, New York. The certified airline transport pilot, commercial rated first officer, flight attendant, and 32 passengers were not injured. An instrument flight rules (IFR) flight plan was filed for the flight destined for Columbia Metropolitan Airport (CAE), South Carolina. Visual meteorological conditions prevailed for the flight conducted under 14 CFR Part 121. The flight crew reported that they were receiving vectors from air traffic control when the numbers 1, 2, and 3 flight displays went blank. Shortly after, the numbers 4 and 5 flight displays went blank. Several seconds later, all five screens returned momentarily before they failed again. As the flight continued, information displayed on all five screens was intermittent. Eventually, all five screens returned to normal and displayed the correct information. When the flight displays returned to normal, red "X's" were displayed on the air data computer (ADC) and the attitude heading reference systems (AHRS). Additionally, yellow crew alerting system (CAS) messages were observed and chimes were heard during the loss and return of flight display information. Also during this event, the number 1 radio management unit (RMU) went blank for approximately 3 seconds, and the number 2 RMU went blank for approximately 1 minute. When the number 2 RMU came back on-line, the test mode was engaged. The captain inspected the overhead panel and could not detect an electrical problem, and there was no sign of smoke. The crew declared an emergency, returned to La Guardia Airport and landed uneventfully. This was the operator's fourth reported instance where a loss of flight displays had occurred in-flight for this make/model airplane. Based on previous testing parameters that were conducted on two other aircraft , only the bus power control unit (BCPU) was examined. The BCPU was shipped to the manufacturer, Thales Avionics Electrical Systems, for examination and testing. The testing was performed under the supervision of the Bureau Enquetes - Accidents (BEA), with representatives from the Civil Aviation Authority-United Kingdom (CAA-UK) and Fairchild Dornier also participating. A review of maintenance records showed three previous repair orders. Visual examination of the unit found traces of varnish on the J1A and J1B pins. When the unit was energized, both the 10PPA and the 10PPB contactors closed; only the 10PPA should be closed. All other tests were successfully completed with no discrepancies noted. The unit was then heated to 70 degrees C (158 degrees F), the failure was able to be repeated but was intermittent. Disassembly revealed a broken transistor (Q21). The unit was repaired and then tested with no discrepancies found. As a result of the investigation, no failure could be identified that would have caused the loss of flight displays. In addition, the manufacturer reviewed the design of the electrical system and could not identify a design failure. Weather reported at LGA, at 1351, included wind from 300 degrees at 11 knots, visibility 10 statute miles, scattered clouds 5,000 feet, broken clouds 15,000 and 25,000 feet, temperature 84 degrees F, 62 degrees F, and altimeter setting 29.79 inches HG.