AFTER FAILING TO PLACE THE GEAR LEVER IN THE OFF POS AFTER RETRACTION; FLT CREW OF B737-300 DO SO AT FL220 AND INADVERTENTLY EXTEND THE GEAR. ALTDEVS ENSUE WHEN SPD ADJUSTMENTS ARE MADE TO ALLOW GEAR CYCLING.

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WE HAD REACHED CRUISE ALT (FL220) AND 300 KTS ON OUR LAST LEG OF A 6 LEG DAY HOME. I; THE PF; LOOKED OVER AND NOTICED THAT THE LNDG GEAR WAS IN THE UP POS. THIS ALL MAY HAVE ENDED HERE IF I'D JUST ASKED THE PM TO PLACE THE HANDLE IN OFF. HOWEVER; WHAT I DID WAS REACH OVER AND LOWER THE GEAR HANDLE. THE HANDLE WAS A LITTLE HARD TO MOVE AND WENT EVER SO SLIGHTLY PAST OFF AND THE MAIN GEAR RELEASED. I IMMEDIATELY MOVED THE HANDLE BACK TO THE UP POS. THERE WAS SOME NOISE AND BOTH MAIN GEAR UNSAFE (RED) LIGHTS WERE ON. THE ACFT STARTED CLBING (300 FT) SO I DISENGAGED THE AUTOPLT; REDUCED PWR AND GOT BACK ON ALT. A QUICK ASSESSMENT OF THE SIT SAID WE NEEDED TO SLOW BELOW GEAR UP SPD AND CYCLE THE LNDG GEAR. NOT DOING THIS EVERY DAY AT FL220; DURING THE SLOW DOWN/LNDG GEAR CYCLING WE LOST 600 FT. ATC ASKED US ABOUT BOTH DEVS. THERE WAS NO CONFLICT AND WE GOT RIGHT BACK ON ALT. THE LNDG GEAR CYCLING WORKED; THE LIGHT WENT OUT AND WE WENT ON TO DAL. I HAD A CALL FROM THE 'A' FLT ATTENDANT IMMEDIATELY AFTER THE LNDG GEAR CYCLING. A COUPLE OF OUR 7 PAX WERE CONCERNED. I MADE A PA. IN HINDSIGHT I SHOULD HAVE DONE THE PA BEFORE I CYCLED THE GEAR. THE ACFT WAS WRITTEN UP IN DAL. THE DEVS WOULD NOT HAVE HAPPENED WITHOUT THE LNDG GEAR INCIDENT. THE LNDG GEAR INCIDENT WOULD PROBABLY NOT HAVE HAPPENED IF THE FO; WHO IS IN A BETTER POS AT SUCH A CRITICAL TIME; HAD MOVED THE LNDG GEAR HANDLE. THE PAX (AND FLT ATTENDANTS) WOULD NOT HAVE BEEN AS UNEASY IF THE CAPT HAD MADE A MORE TIMELY PA.