I would reset the DCS51 to factory defaults by closing Option Switch 39. Procedure is in the manual under DCS51 Option Switches.
Would also check the terminal to see if the screws or wire contacts are intact. Also check the power supply. Unlike the DCS50, it needs a DC supply and should measure at least 13.8 volts. possible that the power supply may be drooping. Center pin on plug is positive.
You can swap the seven pin connector from your old DCS50 but do not swap the DCS50 power supply. It is AC and will not work with a DCS51.
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I ordered a DCS51 for the NG portion of our club layout. We had been running the layout with a DCS50 but wanted the extra features supplied by the DCS51. Our other intention was to connect the DCS50 as a booster to the DCS51 on the loconet and run our stationery decoders that operate our tortoise switch machines. That way, the turnouts are isolated from short circuits on the track. Everything went well until I did the "quarter" test on the track to make sure the booster held up the stationery decoders. As it turns out, the short circuit test only recovers properly about 40% of the time.
I removed the booster and tested again with the same results. I removed the DCS51 from the track and retested with two 12" pieces of 14 gauge wires, attached to the track connections on the rear connector. I had the same result. I moved the unit to another house outlet and still had the same problem. I even took the test scenario to a friends house to make sure it wasn't an incoming power issue. Same results.
When I short the screw terminals directly at the connector, it works correctly every time. I have sent the DCS51 back to Digitrax 3 times with no luck. In fact, they sent a brand new unit back the second time and the new unit does the same thing. They claim they don't see any problem with the overload circuit. I asked them if they used the same method I described to them to test the product and never got a clear answer.
Have any of you seen this problem? Try it with your DCS51's. Try it 10 times bac-to-back and see how many times it passes. A good test will show a "walking 0's" pattern in the display and the track power will stay on after the short is removed. When it fails, the display will show 8888 and the unit will power down, forcing you to push the power button to restore normal operation.