IFR TMB7 AT FL180 WAS UNABLE TO MAINTAIN ASSIGNED ALT BECAUSE OF TURB.

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MY FLT ORIGINATED IN SANTA MONICA; CA; WITH A DEST OF JEFFERSON COUNTRY ARPT; CO. SCATTERED TO WIDELY SCATTERED TSTMS WITH TOPS TO FL450; WITH LOWER CUMULO FORM CLOUDS WITH TOPS TO FL300; REQUIRED DEVS L AND R OFF COURSE; BEGINNING IN S CENTRAL UTAH AND CONTINUING INTO DENVER AREA. IN THE VICINITY OF ASPEN; CO (ASE); AT FL290; I WAS CLRED TO BJC VIA 'DIRECT POWDR INTXN THOMSON FIVE ARR.' AS ATC BEGAN TO STEP ME DOWN; DEVS AROUND BUILDUPS AND AREAS OF HEAVIER PRECIPITATION WERE REQUIRED ALMOST CONSTANTLY. I WAS RELYING ON RADAR ON BOARD; AS WELL AS STORM SCOPE. AS I DSNDED THROUGH FL240; I WAS IN AND OUT OF IMC; IN MODERATE TURB. THE RADAR WAS SHOWING WIDE AREAS OF MODERATE PRECIPITATION; BUT NO LIGHTNING. WHILE DEVIATING R OFF COURSE AND LEVELING AT FL180; I ENTERED A CLOUD WHOSE TOP APPEARED TO BE IN THE HIGH 20'S (FT); AND WHOSE BASE APPEARED TO BE 1000-2000 FT BELOW ME. IMMEDIATELY UPON ENTERING IMC AGAIN; I ENTERED MODERATE-TO-SEVERE TURB; VERY HVY PRECIPITATION (RAIN OR POSSIBLY HAIL); MY AUTOPLT DISCONNECTED; AND THE VERT SPD INDICTOR SHOWED A DSCNT RATE IN EXCESS OF 2500 FPM. I APPLIED FULL THROTTLE (I HAD PREVIOUSLY SLOWED THE PLANE DOWN TO MY TURBULENT AIR PENETRATION SPD OF 158 KIAS) IN AN ATTEMPT TO CLB BACK UP TO MY ASSIGNED ALT OF FL180. WHEN I WAS UNABLE TO IMMEDIATELY GET A POSITIVE RATE OF CLB; I ASKED THE CTLR 'FOR LOWER ALT.' HE ORDERED ME TO MAINTAIN FL180; AND WITHIN A FEW SECONDS I BEGAN TO EXIT THE HEAVIEST PRECIPITATION AND CLB BACK UP TO 18000 FT MSL. ACCORDING TO THE DENVER APCH CTLR; A LOSS OF SEPARATION HAD OCCURRED. I BELIEVE THAT THE INTENSITY OF THE RAIN IN THE CUMULUS CLOUD I ENTERED HAD CAUSED THE WINDSHEAR OR DOWNDRAFT I WAS CAUGHT IN. HAD I BEEN ABLE TO MAINTAIN VMC; I WOULD HAVE SEEN THE HVY DOWNPOUR; AND WOULD HAVE AVOIDED THE CLOUD ALTOGETHER. HOWEVER; MY ALT PLACED ME RIGHT IN THE MIDST OF AN AREA OF CLOUDS AND PRECIPITATION; AND MAINTAINING VMC WAS NOT POSSIBLE.