Frequency Pulling. The aircraft antenna, the cables and connectors, the quality of the grounding near the antenna and variations in the boxes make it fairly common that one combination of these may be off spec on frequency. Not only will this be noticed on the ramp test equipment, but the ATC equipment around the country has different tolerance and so operations may experience what seems intermittent behaviour when it works in one region and not another. All shops will centre the frequency of the equipment. If you test a box out of tolerance be sure to note how many megaHertz high or low it tested when you return the box. Technicians can often set it to accomodate the shift of your aircraft. It may also indicate that you have corrosion under the antenna, or damage to the coax cable and/or connectors. A visual check is in order.