B737-300 CREW HAD ALT OVERSHOOT AND UNDERSHOOT BECAUSE AUTOPLT WAS NOT IN THE CORRECT MODE FOR AUTO LEVELOFF IN ZNY CLASS E AIRSPACE.

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THE FLT WAS CONDUCTED IN ACTUAL IMC FROM MDT TO ABE. TOTAL ENRTE TIME WAS 18 MINS. ON CLBOUT; WE WERE ASSIGNED AN ALT OF 7000 FT. I (CAPT) WAS THE PF. AT 7000 FT; I SAW THAT THE ACFT WAS STILL CLBING AND MADE AN IMMEDIATE CORRECTION; BUT WE DEVIATED ABOVE; THEN BELOW THE ASSIGNED ALT; BEFORE STABILIZING AT 7000 FT. THE FO RPTED TO ATC THAT WE WERE EXPERIENCING A PROB WITH THE AUTOPLT AND WERE CORRECTING BACK TO ALT. I DETERMINED THAT THE REASON THE ACFT DID NOT CAPTURE THE ALT WAS BECAUSE IT WAS NOT IN THE COMMAND MODE; BUT THE CTL WHEEL STEERING MODE AT THE TIME OF LEVELOFF. I ENSURED THAT THE AUTOPLT WAS ENGAGED IN THE COMMAND MODE AND THE REST OF THE FLT WAS CONDUCTED IN A NORMAL FASHION. ON THE GND IN ABE; I REVIEWED THE FLT AND CONCLUDED THE MOST LIKELY CAUSE OF THE ALTDEV WAS DUE TO A 'SWITCHOLOGY' ERROR ON MY PART (IE; I THOUGHT I HAD SELECTED THE COMMAND MODE; BUT IN FACT HAD NOT). CONTRIBUTING FACTORS INCLUDE HIGH WORKLOAD ASSOCIATED WITH A SHORT FLT CONDUCTED IN IMC AND CONCLUDING WITH AN ILS AT DEST. THE RWY IN USE AT MDT WAS RWY 13 AND THE LNDG RWY IN ABE WAS RWY 6 WHICH FURTHER COMPRESSED THE AVAILABLE TIME. ALSO; THE PRESSURIZATION WAS OPERATED IN THE STANDBY MODE BECAUSE OF A MAINT CARRYOVER AND ADDED TO THE WORKLOAD. IN CONCLUSION; THE ERROR OCCURRED BECAUSE I WAS EVIDENTLY DISTR BY ANOTHER EVENT AND WAS NOT CONCENTRATING ON THE TASK AT HAND. I WILL TRY TO EXERCISE MORE VIGILANCE IN THE FUTURE.