B737 CREW EXPERIENCES POPPING AND SMOKE IN COCKPIT. EMER LNDG AT DEP ARPT; PHX.

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PASSING 15000 FT WE HEARD A POPPING NOISE BEHIND FO'S CIRCUIT BREAKER PANEL; WENT AWAY FOR A MIN; THEN CAME BACK. THIS TIME I SAW SPARKS AND SMOKE COMING FROM GAP IN CIRCUIT BREAKER PANELS. WE PUT ON OXYGEN MASKS AND DECLARED EMER AND RETURNED TO PHX. CIRCUIT BREAKER POPPED (#2AC) AND SPARKING STOPPED. WE DID NOT PREPARE THE CABIN AS THE SPARKING STOPPED; HOWEVER; WE BRIEFED THE FLT ATTENDANTS. CHKLISTS WERE ACCOMPLISHED AND LNDG WAS UNEVENTFUL AND PLANE WAS TAKEN OTS. SOME PARTS WERE CHANGED AND NEXT DAY WE WERE TO FERRY THE PLANE WITH MAINT ABOARD. THE TKOF WAS NORMAL; BUT PASSING 14000 FT; THE POPPING STARTED AGAIN -- THIS TIME NO SMOKE AND SPARKS; BUT WE DECIDED WE WEREN'T GOING TO WAIT FOR IT TO START SO WE IMMEDIATELY RETURNED TO PHX. LNDG UNEVENTFUL. RPTR STATES THAT THE ACFT INVOLVED IS A B737-300. HE STATES THAT MAINT COULD FIND NO OBVIOUS REASON FOR THE SMOKE AND POPPING SOUNDS PRIOR TO THE SECOND FLT WHICH WAS TO BE A FERRY WITH MAINT PERSONNEL ON BOARD. AFTER THE SECOND FLT RETURNED TO PHX THE MAINT PERSONS DETERMINED THAT THE PROB WAS WITH A BUNDLE OF WIRES IN THE CIRCUIT BREAKER PANEL ON THE FO SIDE; WHERE SOME INSULATION HAD WORN AWAY AND THE WIRES WERE ABLE TO TOUCH EACH OTHER AS WELL AS THE AREA ADJACENT TO THEM IN THE PANEL. AS FAR AS THE RPTR KNOWS; THE ACFT IS BACK IN SVC. THE POPPING SOUND WAS NOT HOWEVER; THE SOUND OF BS POPPINGS.