A CANADAIR CL600 FLIES TOO SLOW AT HIGH ALT FOR A HOLDING PATTERN AND ENCOUNTERS BUFFETING INDICATING ACFT IS APCHING A STALL. FLC DSNDS ACFT TO REGAIN SPD IN ZJX; FL; AIRSPACE.

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IN 20 YRS OF FLYING CPR JETS; INCLUDING 10 IN THIS AIRFRAME; I HAVE NEVER HELD AT FL390. ON MMU-PIE FLT; WE SLOWED 50+ NM N OF TLH WHILE DIVERTING AND EXPECTING DELAY DUE TO SEVERE TSTM LINE AT DEST. DISCUSSED HOLDING PATTERN (DIRECT ENTRY) AND SPD; INPUT HOLD IN FMS; AND PLANNED ON 190 KIAS HOLD. 20 MI LEGS GIVEN AT 20 DME OUTBOUND ACFT FAILED TO TURN; SO I RESELECTED HEADING MODE; AND INITIATED TURN MANUALLY; ADDING PWR AS KIAS WAS ABOUT 180 KTS. DUE TO HIGH WINDS ALOFT; OUTBOUND CRAB ANGLE WAS 60 DEGS. FMS AND DME DISPLAYS DID NOT AGREE; AND WE DISCUSSED THIS AS I CONTINUED THE LONG TURN TO INTERCEPT INBOUND; CONCERNED THAT WE STAY ON HOLDING TRACK: KIAS WAS ABOUT 175 KTS; SO I ADDED MORE PWR. AS THE ACFT LEVELED; I NOTED AIRSPD AT 170 KTS AND FELT AIRFRAME 'RUMBLE.' I ADDED STILL MORE PWR; YET AIRSPD SLOWED TO 165 KTS; AND THERE WAS A SECOND; STRONGER RUMBLE. EXTREMELY CONCERNED; I ALERTED SIC AS I DISENGAGED AUTOPLT; ADDED MAX CLB PWR; AND ALLOWED DSCNT SO AS TO MAINTAIN AIRSPD. AT START OF DSCNT STICK SHAKER ACTIVATED BRIEFLY. AT 38200 FT WE STABILIZED; ADVISED CTR; AND CLBED TO FL390; RETURNING TO NORMAL OPS. ATC EXPRESSED NO CONCERN. CALCULATED STALL FOR CONDITIONS 127 KTS. LOWEST KIAS SEEN: 165 KTS. FACTORS: DISTR FROM FMS. VERY LONG DECELERATION AND ACCELERATION TIMES AT FL390. LATER REALIZED FMS 'DME' INCLUDES HOLDING TRACK DISTANCE (NOT MENTIONED IN MANUAL). EVEN AT SEA LEVEL TKOF THRUST N1% SETTING MAY NOT BE SUFFICIENT TO ACCELERATE AT FL390. FIRST TIME HOLDING/MANEUVERING AT FLT LEVEL; NOT COVERED IN AFM; OP MANUAL OR RECURRENT TRAINING. STAYING 'BEHIND PWR CURVE' WITH INCREMENTAL PWR ADVANCES IN A HIGH DRAG; HIGH ALT STATE. DID NOT RECOGNIZE BUFFET IMMEDIATELY; FELT 165 KIAS TOO HIGH FOR IT. POSSIBLE SHEAR/SUBSIDING AREA BEHIND STRONG TSTM LINE. NO USABLE ANGLE OF ATTACK SYS (ANGLE OF ATTACK FOR STALL WARNING ONLY). TOO CONCERNED ABOUT FUEL -- LANDED 4800 LBS -- (2+24 @ 250 KTS) EVEN AFTER 30 MIN DELAY. THE ONE RIGHT THING DONE: FLEW THE AIRPLANE FIRST; AND DID NOT ALLOW FURTHER DECELERATION. DISTR AND ATMOSPHERIC CONDITIONS MAY HAVE PLAYED SMALL ROLES; BUT PRIMARILY THE SIT WAS DUE TO AN INCOMPLETE UNDERSTANDING OF HIGH ALT; LOWER AIRSPD FLT AND MANEUVERING; AND A LACK OF TRAINING AND GUIDANCE IN THOSE AREAS. ON A LATER FLT AT 3000 FT MSL; 180 KIAS; I OBSERVED THE PWR CURVE/PITCH DYNAMICS; BUT IT IS NOTHING LIKE FL390 DUE TO THE LARGER PWR RESERVE AVAILABLE. THIS REALLY WAS A BLIND SPOT IN MY KNOWLEDGE AND TRAINING; AND COULD HAVE HAD VERY DIRE CONSEQUENCES.