SMA HAS HSI INTERMITTENT FAILURE.

Narrative

ALT AND HDG DEV FROM ASSIGNED DUE TO HSI FAILURE IN HVY WX. CONDITIONS WERE IMC WITH HVY RAIN AND LIGHT TURB. STORM SCOPE SHOWED HVY ELECTRICAL ACTIVITY NEARBY AND LIGHTENING FLASHES OCCASIONALLY VISIBLE. NAV FLAG CAME ON AND THEN WENT OFF; ONLY TO COME BACK ON FOR GOOD A FEW MINS LATER. I THEN DISCONNECTED AUTOPLT AND WAS HAND FLYING THE PLANE WHILE TRYING TO DIAGNOSE THE EXTENT OF THE PROB AND COPY A ROUTING CHANGE FROM CTR. WHILE DOING SO; I ALLOWED THE ALT TO DEVIATE ABOVE ASSIGNED. CTR CTLR QUESTIONED THE ALT AND I TOOK CORRECTIVE ACTION. I ALSO INFORMED CTR OF HSI FAILURE AND MY REQUEST TO DEVIATE TO A DIFFERENT LOCATION FOR LNDG. CTR SUGGESTED ONE LOCATION WHICH I DECLINED AS I HAD CHKED THE LCL ATIS AND DETERMINED THE CEILING TO BE TOO LOW FOR A NO GYRO APCH. FLT CONCLUDED UNEVENTFULLY WITH A LNDG AT GREGG COUNTY ARPT. IN REVIEWING THE CIRCUMSTANCES; I THINK I SHOULD HAVE BEEN QUICKER TO NOTIFY CTR AND COVER THE INST AND SAVE THE TROUBLESHOOTING FOR A FEW MINS; WHILE I CONCENTRATED ON FLYING THE PLANE AND CHANGING MY SCAN TO ADJUST FOR THE MALFUNCTION. NOTIFYING CTR SOONER MAY HAVE HELPED ELIMINATE THE ADDITIONAL PROB OF COPYING A NEW ROUTING AT A TIME WHEN I ALREADY HAD MY HANDS FULL.