PLT OF A MOONEY M28K (MO2K) OVERSHOT ASSIGNED ALT ON CLB CAUSING LTSS WITH ANOTHER ACFT THAT WAS CLRED TO DSND THE ALT ABOVE.

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I WAS ON AN IFR FLT PLAN UAO TO SUN RIVER (S21) ON THE EASTERN SIDE OF THE CASCADES. I FILED IFR BECAUSE OF A MARINE FOG LAYER W OF THE CASCADES THAT EXTENDED FROM 3000 FT TO 5000 FT. I PLANNED TO CANCEL IFR AND USE FLT FOLLOWING AFTER CLRING THE FOG LAYER. ON CLBOUT I WAS GIVEN MULTIPLE ASSIGNED ALTS; THE LAST ONE BEING 9000 FT. AT THIS TIME THE WX WAS VMC ABOVE THE MARINE LAYER AND I WAS ABOUT TO CANCEL IFR. BEFORE THIS WAS ACCOMPLISHED I INADVERTENTLY LET MY PLANE CONTINUE ITS CLB TO 9500 FT. PDX APCH HAD CLRED ANOTHER PLANE IN THE GENERAL AREA TO DSND TO 10000 FT. I AND MY PAX DID NOT EVER SEE THIS OTHER PLANE; DESPITE CLR CONDITIONS. THE OTHER PLANE DID NOT NOTE US EITHER ACCORDING TO RADIO COMS. THE CTLR; HOWEVER; STATED WE HAD PRODUCED A POSSIBLE CONFLICT IN ALT SEPARATION. THE PROB WOULD HAVE BEEN AVOIDED IF I HAD BEEN MORE DILIGENT AND ATTENTIVE TO THE ASSIGNED ALT. I LET MYSELF BE SIDETRACKED BY THE CLR SKIES; BY MY INTENT TO SWITCH FROM IFR TO VFR; AND MY OVERRELIANCE ON ATC.