EMS PLT WANDERED OFF OF ASSIGNED ALT DURING IFR IN IMC DUE TO THE AUTOPLT ALT HOLD NOT HOLDING ALT UNDER SINGLE PLT IFR OPS.

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I LEFT ABC HOSPITAL AT APPROX XX00 HRS ON OCT/WED/96; ENRTE TO XYZ HOSPITAL WITH A PATIENT ON BOARD. I HAD CALLED AND RECEIVED XYZ WX AT APPROX XX40 AND WX WAS 3000 AND 10 PLUS VISIBILITY (THIS IS AN ESTIMATION) AS I FLEW SEBOUND CLBING THROUGH ABOUT 1500 FT MSL; THE WX STARTED TO DETERIORATE. I CALLED CTR AND RECEIVED A CLRNC TO 4000 FT MSL (HIS RADAR VECTORING ALT) AND A SQUAWK). HE TOLD ME TO GO DIRECT TO XYZ. I ASKED IF HE WANTED ME TO JOIN V144 TO DO THIS AND HE SAID NO; TO GO DIRECT. I WAS AT ABOUT 3000 FT MSL AT THIS TIME. MY ATTN WAS DIVERTED AT THIS TIME TO FLYING THE ACFT BECAUSE OF SOME AUTOPLT PROB. (WE ARE SINGLE PLT IFR) AS I GOT TO 4000 FT I LEVELED OFF AND CONTACTED YYY CTR; RPTED LEVEL AND INFORMED CTR I WAS UNABLE TO NAV DIRECTLY TO XYZ BECAUSE OF GPS WAS VFR ONLY. I ASKED TO FLY THE APE VOR BECAUSE I WAS RECEIVING A RELIABLE SIGNAL FROM IT AND COULD FLY DIRECT TO THERE. CTR SAID ROGER THAT WOULD BE OKAY AND PROCEED AS REQUESTED. AT THIS TIME MY AUTOPLT MALFUNCTIONED CAUSING A CLB TO 4500 FT MSL. I FIXED THE MALFUNCTION AND DSNDED TO 4000 FT MSL AGAIN. I WAS ABOVE 4000 FT FOR APPROX 2 MINS. I WAS HANDED OFF TO XYZ APCH AT 4000 FT MSL THEY DSNDED ME TO 3500 FT MSL. I BROKE OUT AT 3800 FT MSL WITH SOME SCATTERED CLOUDS BELOW ME; THEN WAS GIVEN 3000 FT MSL I WENT BELOW THE LAST SCATTERED LAYER AROUND 3200 FT MSL. I CONTINUED ON; GETTING LOWER ALTS AS HIS MINIMUM VECTORING ALT PERMITTED. FINALLY ABOUT 10 OR SO MI FROM MY DEST I CANCELED MY IFR AND CONTINUED VFR WITH NO FURTHER INCIDENT. I BELIEVE THAT THIS PORTION OF THE COUNTRY COULD USE BETTER RADAR COVERAGE. AS YOU GET CLOSE TO THE LIMIT OF ANY CTR'S AIRSPACE RADAR COVERAGE DETERIORATES. MOST HELIS FLY BELOW 4000 FT AT ALL TIMES AND IT IS VERY HARD TO GET RADAR COVERAGE FOR US.