PIPER P46 PLT HAS AN ALT EXCURSION DURING DSCNT FOR LNDG AT GJT.

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I WAS UNDER AN IFR FLT PLAN AND WAS MAKING AN APCH INTO GJT; CO. WX CONDITION WAS CLR; LIGHT TO MODERATE TURB ON THE DSCNT FROM 16000 FT TO MY NEW ASSIGNED ALT OF 12000 FT AND I WAS SLOWING THE DSCNT RATE AND SPD ANTICIPATING THE USUAL INCREASE IN TURB AT THE LOWER ALT. I WAS ON A HDG OF APPROX 330 DEGS WITH GJT APPROX 20 MI AHEAD. EITHER BEFORE OR AFTER MY AUTOPLT CAPTURED 12000 FT I HAD PRESET INTO THE AUTOPLT COMPUTER; THE ACFT PITCHED UP SUDDENLY. I BEGAN AN INST SCAN AND SAW THE AIRSPD DROPPING. I SIMULTANEOUSLY DISCONNECTED THE AUTOPLT AND PUSHED THE YOKE FORWARD WHILE TRIMMING NOSE DOWN. TURB STRONGLY INCREASED AND I TRIED TO GET THE ACFT SPD INTO THE 'MANEUVERING SPD' AREA TO MINIMIZE STRUCTURAL LOADS AND ALSO WORKED TO MAINTAIN PITCH AND ROLL STABILITY. (THE ACFT PITCHED UP BECAUSE I ENTERED A DOWNDRAFT AND THE AUTOPLT; ATTEMPTING TO CORRECT AND MAINTAIN ALT; THAT WAS PRESET; TRIMMED THE ACFT INTO A NOSE-UP ATTITUDE. THIS IS AN EVENT I HAVE TRAINED FOR AND AM FAMILIAR WITH.) I IMMEDIATELY USED OUTSIDE VISUAL REFS TO ENSURE TERRAIN AND ACFT SEPARATION; AND ORIENTATION FOR ROLL CTL AND PITCH CTL AND REFED THE AIRSPD TO AVOID STALL/SPIN BY MAINTAINING SUFFICIENT AIRSPD BY PITCHING DOWN. BECAUSE OF THE STRONG TURB AND MY CONCERN OF EXCESSIVELY LOADING THE ACFT STRUCTURE; AND BECAUSE I WAS LOOKING AT TERRAIN AND THE TERRAIN WAS NOT SUFFICIENTLY CLOSE TO POSE ANY THREAT; I DID NOT INCREASE G-LOADING TO CLB AND CONCENTRATED ON MAINTAINING THE ACFT IN CTLED FLT. MOMENTARILY APCH CTL CALLED AND ASKED MY ALT. I REPLIED 10000 FT AND I HAD ENCOUNTERED A DOWNDRAFT. THE CTLR ADVISED ME TO TURN 90 DEGS L. I COMPLIED IMMEDIATELY AND FLEW OUT OF THE STRONG TURB.