PLT CLBED THROUGH ASSIGNED ALT BY 300 FT.

Narrative

AFTER DEP FROM ALTOONA; I ENTERED OVCST LAYER AT 900 FT AGL. SHORTLY AFTER ENTERING IMC; MODERATE RIME ICE WAS EXPERIENCED. THIS WAS THE FOURTH BUILDUP OF RIME ICE WITHIN THE LAST 2 HRS (3 APCHS; 1 OF WHICH WAS A MISSED AT JOHNSTOWN; PA; AND 1 CLBOUT AT ALTOONA). DEICE BOOTS WERE ACTIVATED 2 TO 3 TIMES ON CLBOUT. UPON REACHING THE TOP OF THE OVCST LAYER; APPROX 4700 FT MSL; MY ATTN WAS DIVERTED TO SHEDDING THE REMAINING ICE; REACTIVATING THE WING BOOTS; AND RE-VERIFYING PITOT AND WINDSHIELD DEICE WERE FUNCTIONING CORRECTLY WAS ENOUGH TO DISTRACT ME TO CLB THROUGH 5000 FT MSL TO APPROX 5300 FT MSL. ATC CLRNC RESTR CLB TO 5000 FT. UPON NOTICING THE ERROR AT 5300 FT IMMEDIATE CORRECTIVE ACTION WAS TAKEN TO DSND TO 5000 FT. THE TIME OF THIS OCCURRENCE WAS THE FIFTH LEG OF A CARGO TRIP. THE LEGS OF THE TRIP BEING XA00 PIT-BED; XB15 BFD- PIT XD00 PIT-JST MISSED; XD45 JST-AOO; XF15 AOO-ESN. APCHS AT BFD; JST; AND AOO ALL HAD MODERATE RIME ICE ON DSCNT. ALL LEGS WERE DONE SINGLE PLT PART 135 IFR.