BE35 PLT HAD ALT EXCURSION IN SHV CLASS C AIRSPACE.

Narrative

WE HAD JUST TAKEN OFF FROM SHV FOR THE RETURN TRIP TO BATON ROUGE. THIS WAS AN AIRPLANE I HAVE A LIMITED AMOUNT OF TIME IN. WE HAD LEVELED OFF AT 5000 FT AND WERE ON AN IFR FLT PLAN. THERE IS NO AUTOPLT ON THIS AIRPLANE. I THOUGHT I HAD ALL THE PWR SET AND THE TRIM SET TO KEEP US LEVEL; BUT APPARENTLY I DIDN'T; AS WE CLBED TO 5500 FT. WHILE I TOOK THE TIME OUT TO TROUBLESHOOT A HEATER PROB WE WERE HAVING; I NOTICED THE DISCREPANCY IN THE ALT. JUST A FRACTION OF A SECOND BEFORE; THE CTLR CALLED ME TO VERIFY MY ALT. I CONFIRMED WE WERE AT 5500 FT AND THAT MY REQUESTED ALT WAS 5000 FT. HE ASKED ME TO GO BACK TO 5000 FT; WHICH I DID. ONCE I GOT DOWN TO 5000 FT; I CALLED THE CTLR BACK AND JOKINGLY SAID; 'I GUESS I HAD A VFR FLASHBACK!' HE SAID THAT I HAD GOTTEN HIS CURIOSITY UP AND WAS WONDERING WHAT I WAS UP TO. A FEW MINS LATER I WAS HANDED OFF TO POLK CTL. THIS PROB AROSE BECAUSE I WAS DISTR TRYING TO FIX A HEATER PROB WITH AN AIRPLANE THAT I HAVEN'T FLOWN IN THE COLD WX BEFORE. I TOOK MY FOCUS OFF MY INSTS LONGER THAN I SHOULD HAVE. AS I TROUBLESHOT MY PROB; I SHOULD HAVE MONITORED MY ALT BETTER; BUT SINCE I THOUGHT I HAD MY PWR AND TRIM SET FOR LEVEL FLT; I LET MY ATTENTIVENESS LAPSE FROM FLYING THE AIRPLANE. A CONTRIBUTING FACTOR WAS THE DIM LIGHT IN THE AIRPLANE WHICH MADE THE TROUBLESHOOTING OF THE HEATER MORE DIFFICULT.