'PAX DOOR OUTER HANDLE' CAUTION LIGHT AND 'OVERBOARD COOL FAIL' CAUTION LIGHT CAME ON IN CLB PROMPTING THE CREW TO RETURN LAND.

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AS WE TAXIED TO THE RWY; ALL THE CHKLISTS WERE COMPLETED AND EVERYTHING WAS OK. WE COMMENCED OUR TKOF AND CLBING OUT OF APPROX 600 FT AGL WE GOT THE 'PAX DOOR OUTER HANDLE' CAUTION MESSAGE. WE CHKED THE PRESSURIZATION AND IT SEEMED NORMAL AND WAS PRESSURIZING. THIS CAUTION MESSAGE HAS BEEN A NUISANCE MESSAGE IN THE PAST. WE CONTINUED TO CLB OUT AND WERE STILL PRESSURIZING. AFTER COMPLETING OUR CLB CHKLIST; WE PROCEEDED TO DO THE PAX DOOR OUTER HANDLE CHKLIST WHICH CALLS FOR US TO CHK THE PRESSURIZATION AND VISUALLY CHK THE DOOR. THROUGH ABOUT 10000 FT WE GOT ANOTHER MESSAGE 'OVERBOARD COOL FAIL' CAUTION WHICH EFFECTS PRESSURIZATION. AT THIS POINT WE STARTED DEPRESSURIZING AND THEREFORE STOPPED OUR CLB AND INITIATED A DSCNT. WE HAD JUST CONTACTED OUR FLT ATTENDANT AND HAD HER CHK THE DOOR. SHE TOLD US THAT 3 OF THE 8 GREEN MARKS THAT INDICATE THE DOOR IS CLOSED; WERE NOT LINED UP AND THAT SHORTLY AFTER TKOF SHE HEARD A MECHANICAL CLUNKING NOISE COME FROM THE DOOR. AFTER TALKING WITH THE FLT ATTENDANT; WE PROCEEDED TO DO THE APPROPRIATE PART OF THE CHKLIST. THIS CALLED FOR US TO GO TO MANUAL PRESSURIZATION AND DSND TO AT LEAST 10000 FT PLUS THEN RETURN FOR LNDG. WE NOTIFIED ATC WE NEEDED TO RETURN TO CVG AND TOLD THE PAX WE RETURNED FOR LNDG WITHOUT FURTHER INCIDENT. HOWEVER; THE FLT ATTENDANT SAID THAT ON SHORT FINAL THE OTHER 5 GREEN DOOR MARKS DISAPPEARED AND THE RED DOOR UNLOCKED FLAG APPEARED. THIS AGAIN; SHE SAID; WAS ASSOCIATED WITH ANOTHER MECHANICAL CLUNKING NOISE WITHIN THE DOOR. ON THE GND; AFTER TALKING TO MAINT; THEY SAID THERE WAS 'QUITE A BIT OF PLAY IN THE DOOR HANDLE' AND 'IT WAS PROBABLY SOMETHING WITHIN THE DOOR MECHANISM' A DAY LATER WHEN DISCUSSING THIS WITH A MECH; HE SAID THAT THERE WAS A HISTORY OF THIS TYPE OF THING HAPPENING. IN CLOSING; ALTHOUGH WE DID THE PROPER PROC IN THE CHKLIST; THE CHKLIST WAS SOMEWHAT CONFUSING TO FOLLOW BECAUSE OF THE WAY IT IS SET UP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SUGGESTS THAT THIS INCIDENT IS THE MANIFESTATION OF AN ACFT THAT WAS NOT ACTUALLY DESIGNED FOR THE MISSION IT IS FLYING. THE ORIGINAL DESIGN WAS FOR A CORPORATE ACFT THAT WOULD FLY 300 OR 400 HRS A YR AND THE DOORS WOULD BE OPENED AND CLOSED A COUPLE OF TIMES A DAY ON THE DAYS IT FLEW. AS A REGIONAL ACR JET THE ACFT FLIES 300 HRS PER MONTH AND THE DOORS ARE OPENED AND CLOSED 10 OR MORE TIMES EVERY DAY OF THE MONTH. CONSEQUENTLY THE LATCHES ARE WEARING LOOSE AND SLOPPY. HE SAID THE DOOR IS NOT EXACTLY A PLUG TYPE DOOR BUT HE BELIEVES THAT THE LATCHES ARE NOT WHAT SECURES THE DOOR FROM OPENING IN PRESSURIZED FLT. NEVERTHELESS; THE FLT ATTENDANT WAS SO TERRIFIED SHE COULD NOT FUNCTION AS THE FLT ATTENDANT ON THE REPLACEMENT ACFT AND THE PAX HAD TO BE OVERNIGHTED AND THE REPLACEMENT ACFT FERRIED EMPTY TO THE DEST. THE CHKLIST CORRECTIVE ACTION FOR THE 'OVERBOARD COOL FAIL' CAUTION LIGHT ON THE GND IS TO CHK THE PAX DOOR FOR PROPER CLOSING. THERE IS NO CHKLIST FOR THAT LIGHT INFLT.