Bally E-2090 slot error code 30...06

[czsm]

New problem on this recently repaired Bally E-2090 slot machine.  Has a new MPU board and EPROM's, courtesy of Barry at foxslots.  It was working perfectly for about 30 coins (A few payouts alongs the way).  I started to test the bookkeeping meter function using the key switch.  No problems there. I then went to test the remaining meters by turning the door key counter-clockwise and then the key switch and the machine hung.  Reset the power several times.  Pushed the reset button on the hopper control board several times.  Same reading on the display every time (almost).  3.0...0.6.   (I get a display of 3.....0.6 occasionally but I think that might just be a bad pin on the ribbon cable going to the display. 

While I was typing this and pondering the problem, for about 15 minutes with the door open, the door open buzzer all of a sudden goes off and the display now reads the normal 50....1.  Closed the door, pulled the handle and it works fine now.

Any ideas on what the problem was/is?