B757 FLT CREW HAS A TRACK HDG DEV DURING MIA DEP FROM MIA.

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NORMAL PREFLT ON FMC AND FLT PLAN; ALL CHKED OK. DURING INITIAL CLB ATC REQUESTS INTERCEPT OF VALLEY TRANSITION (058 DEG RADIAL VKZ). WHEN I SELECT LEGS PAGE; THE RTE IS GONE AND I CANNOT RESELECT ANY POINTS (POINTS DISAPPEAR WHEN SELECTED TO RTE OR LEGS PAGE; EXECUTE LIGHT ILLUMINATES WITHOUT SHOWING POINT). DURING THIS TIME WE PUT THE RAW DATA ON THE MAP DISPLAY TO SHOW THE GREEN LINE OR 058 DEG RADIAL OF VKZ. I TOLD THE CAPT THERE'S THE RAW DATA; AND WENT BACK HEADS DOWN TO FIX THE RTE. THE CAPT STARTED THE TURN EARLY AND END UP PARALLELING THE COURSE TO THE W OF THE TRANSITION BY ABOUT 3 MI. ATC QUERIED US AND AFTER THE CONFUSION WE ASKED FOR A VECTOR AND WERE HANDED OFF TO ZMA. AFTER WE WERE ON THE VECTOR I WENT BACK HEADS DOWN AND FOUND OUT I COULD GET NAV POINTS INTO RTE #2. AFTER VERIFYING OUR POS WITH THE INIT PAGE (GPS COORDINATES) AND WAYPOINT PAGE; WE WERE ON OUR WAY. LATER; I WAS ABLE TO MOVE ALL INFO BACK INTO RTE #1 AND ALL WORKED NORMALLY. WE ALSO NOTICED THAT THE MIA; BOS AND THE COMPANY RTE #2 WERE ALL STILL THERE; THE RTE WAS ALL THAT WAS MISSING. AFTER THE FACT; I SHOULD'VE NOT STAYED HEAD DOWN DURING A CRITICAL TIME OF FLT; AS WELL AS USE THE COMPASS ROSE PAGE ON THE HSI; RATHER THAN MAP WITH THE RAW DATA SELECTED. THE CAPT WAS IN A GREATER RANGE AND THIS IS WHY WE THOUGHT THE ACFT WAS APCHING THE RADIAL; BUT IT WAS ACTUALLY A FEW MI AHEAD. HOWEVER; I'VE NEVER ENCOUNTERED AN FMC ANOMALY LIKE THIS BEFORE. I'VE SEEN THE RTE DROP OUT; BUT IT WAS ALWAYS READY TO BE REACTIVATED. THIS RTE WAS GONE! THE ONLY WAY I KNOW TO PURGE A RTE AFTER IT'S BEEN ACTIVATED IS TO SELECT THE DEST TO THE ORIGINATION ON THE RTE PAGE. SUPPLEMENTAL INFO FROM ACN 700076: I WAS FLYING; BUT DIDN'T REALIZE THAT I HAD CHANGED TO EXPANDED SCALE WHEN LOOKING FOR THE MAGENTA LINE; AND FAILED TO GO BACK TO THE 10 MI SCALE OR EXPANDED MODE WHEN INTERCEPTED THE VOR. THIS CAUSED ME TO TURN W OF COURSE BEFORE CORRECTING FOR THIS. ATC ASKED IF WE WERE INTERCEPTING THE RADIAL; AND AT THAT TIME; I REALIZED I WAS W OF COURSE AND CORRECTED.