ALTDEV ALT OVERSHOT IN DSCNT. PROX OF TSTM ACTIVITY. POTENTIAL CONFLICT WITH XING TFC.

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MANY THINGS IN A SHORT TIME; THEN PUT IT OUT OF MIND AND FLY THE AIRPLANE; THEN HRS LATER RECOUNT WHAT HAPPENED!? DURING THE LAST PART OF CRUISE I RETURNED TO THE COCKPIT FROM USING THE LAVATORY. WE WERE AT 35000 FT AND SAW 25000 SET IN THE MODE CTL PANEL AND QUERIED THE CAPT (PF) WHO SAID OUR CLRNC WAS 'DEV E AROUND WX AT OUR 12 O'CLOCK THEN DIRECT MSY AND TO CROSS....' JUST THEN THE CTLR ASKED US TO START DOWN. HE THEN SAID TO TURN R TO A HDG TO INTERCEPT THE MSY 355 DEG RADIAL AND TO CROSS 50 MI S OF MEI AT 290. AFTER THE CAPT SET IN THE NEW HDG HE SET UP THE FMC TO INTERCEPT THE RADIAL. I LOOKED UP MSY FREQ AND SET UP RAW DATA AS HE WAS SETTING THE COMPUTER. I ASKED HIM IF EVERYTHING WAS SET UP AND THAT I WOULD TRY TO GET THE ATIS AGAIN AND CALL OPS. HE REPLIED IT WAS ALL SET UP. DUE TO WX THE ATIS WAS INTERMITTENT. THE CTLR CALLED TFC TO THE L OF THE ACFT NOSE THAT WAS L TO R AT FL280. (TCASII EQUIPPED BUT RADAR IN A LONG RANGE SETTING.) I LOOKED UP AT THE MODE CTL PANEL AND SAW 28000. I SAID TO THE CTLR THAT WE WERE DSNDING TO FL290. CTLR SAID TO STOP OUR DSCNT AT FL290 (WE WERE ABOUT 29100 FT AND DSNDING IN VNAV PWR OFF. THE CAPT PUSHED SOME BUTTONS ON THE MODE CTL PANEL -- DON'T REMEMBER WHAT ONES OR IN WHAT ORDER). THE ACFT KEPT DSNDING AND THE CAPT REACHED FOR THE CTL WHEEL AS THE CTLR SAID TO TURN R TO A HDG. THE CAPT THEN REACHED UP AND SET THE NEW HDG IN THE MODE CTL PANEL AND SET 290 IN THE MODE CTL PANEL AND PRESSED VNAV. THE ACFT CONTINUED ITS DSCNT AND WE WERE IN PRETTY HVY TURB (THE FIRST R TURN FOR TFC PUT US IN WX). AGAIN; THE CAPT STARTED REACHING FOR THE CTL WHEEL AND AGAIN A FURTHER R TURN WAS ISSUED AND THE CAPT AGAIN SET THE NEW HDG. ON MY PART; THE CAPT WAS AWARE OF US GOING BELOW FL290; SO TO HAVE VERBALLY SAID ANYTHING TO THAT END I FELT WOULD HAVE ADDED TO HIS LOAD. TWICE I WAS ABOUT TO REACH FOR THE CTL WHEEL BUT JUST AS I WAS ABOUT TO; HE DID. AS HE WAS SETTING IN THE SECOND HDG I REACHED UP ENGAGED VERT SPD AND SPUN THE VERT SPD THUMB WHEEL TO A LARGE POSITIVE RATE. I THEN NOTICED OUR TURN RATE WAS SLOW AND LOOKED OVER AT THE BANK LIMITER AND IT WAS SET AT THE SECOND NOTCH! (12 1/2 DEGS). I SET IT AT 25 DEGS (I DIDN'T GO TO 30 DEGS BECAUSE OF THE TURB). THE TCASII NEVER WENT OFF BUT WE WERE TOLD BY NEXT CTLR TO CALL QUALITY ASSURANCE ON THE GND. THEY SAID RADAR SHOWED WE HAD GONE TO 28500 FT AND ONLY HAD 4.5 MI. COCKPIT COORD: DOUBLE-CHK EVERYTHING CAPT DOES AND WHY. (THIS CAPT WAS EXPERIENCED IN THIS ACFT.) COMPANY POLICY: BEFORE I ACTED; TWICE I ALMOST TOOK OVER. WE NEED COMPANY POLICIES THAT MANDATE THE VARIATIONS ALLOWED BEFORE THE FO MUST TAKE OVER. (I KNEW IN MY MIND WE WEREN'T GETTING TO FL280 AT ANY POINT). CTLR: GIVING CLRNC TO NAV USING 1 NAVAID WHILE GIVING DSCNTS BASED ON A DIFFERENT ONE SHOULD BE BANNED. THE CTLR NEVER USED THE WORD IMMEDIATE IN OUR TFC TURN CLRNC.