A320 FLT CREW RECEIVES A WINDSHEAR ALERT; EXECUTES A GAR; HAS AN ALTDEV AND DIVERTS TO ANOTHER ARPT.

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TSTMS WERE APCHING IAD FROM THE W AS WE BEGAN AN APCH TO RWY 1L. WE RECEIVED A WINDSHEAR REPORT FROM A B777 IN FRONT OF US ON THE APCH WHEN WE WERE APPROX 3000 FT AND DSNDING ON THE GS. THE SHEAR REPORTED WAS IN EXCESS OF 20 KTS AND WE ELECTED TO ABANDON THE APCH. WE WERE ABOVE 2000 FT AND IAD APCH GAVE US A HDG AND A CLRNC TO 2000 FT FOR THE MISS. AFTER INITIALLY ACCEPTING THE CLRNC WE ASKED FOR 3000 FT TO AVOID DSNDING IN THE MISSED APCH. WE WERE GIVEN 3000 FT ALT CLRNC AND THE FO SELECTED AUTOPLT ON TO REDUCE WORKLOAD. THE FLT DIRECTOR FMA SHOWED ALT AFTER WE HAD SELECTED 3000 FT AND PULLED THE ALT KNOB. THE ACFT (IN AN APPARENT UPDRAFT) CLBED TO 3300 FT BEFORE FO DISCONNECTED THE AUTOPLT AND RECAPTURED THE CLRED ALT MANUALLY. IT WAS A VERY HIGH WORKLOAD TIME AND WE WERE DECIDING TO DIVERT TO AVOID THE ENCROACHING WX. SOME OF MY CONCERNS RAISED BY THIS EVENT: THE 2500 FT GPWS ALERT WAS SO LOUD IT INTERRUPTED THE FIRST PLT REPORT OF THE SHEAR -- I HAD TO REQUEST THE CTLR TO REPEAT THE REPORT. THIS COST VALUABLE TIME AND RADIO XMISSIONS. THE HIGH VOLUME OF THE GPWS ALERTS BLOCK OUT ALL RADIO TFC. THE CLRNC TO MISS AT 2000 FT SEEMED INAPPROPRIATE GIVEN THE HEIGHT WE INITIATED THE MISS. I SHOULD NOT HAVE ACCEPTED THE ALT PORTION OF THE CLRNC AND IMMEDIATELY REQUESTED 3000 FT. EVEN A 'SOFT GAR' IS A HANDFUL WHEN YOU ARE INITIATING A MISSED APCH SO CLOSE TO THE ASSIGNED ALT. THE AUTOPLT FAILING TO HOLD ALT AFTER WHAT WE BOTH BELIEVED TO BE AN APPROPRIATE SELECTION AND ALT MANEUVER ADDED TO OUR WORKLOAD.