AN ACR LGT IS UNABLE TO MAINTAIN CRUISE SPD AND ALT AT FL310 FOR NO APPARENT REASON. PIC HAS TO DSND TO PREVENT STALL BUFFET.

Narrative

SEVERAL MINS AFTER FLYING THROUGH AN OVERHANG FOR ABOUT 3 MINS IN RELATIVE SMOOTH AIR; IN CRUISE; VFR; AT FL310; AIRSPD WAS SLOWING. ALL OTHER ENG INDICATION NORMAL. NO INDICATION OF POSSIBLE ICE ON WINDSHIELD OR WIPERS OR ENG READINGS. DID NOT NOTICE RAT TEMP. PITOT HEAT ON AND NORMAL. SPD WAS NOW DOWN TO ABOUT 235 MPH AND ACFT STARTED TO BUFFET. AUTOPLT AND AUTOTHROTTLE STILL ON. TURNED ON ENG AND AIRFOIL ANTI-ICE. SPD DETERIORATED FURTHER. DISENGAGED AUTOPLT AND AUTOTHROTTLE. LOWERED NOSE TO AVOID BUFFETING OR POSSIBLE STALL. AS SOON AS CTR FREQ WAS CLR WE ADVISED CTR OF OUR SIT (UNABLE TO HOLD ALT AND IN A SLOW DSND). CTR ASKED IF WE COULD LEVEL OFF AT FL290 AND TO ADVISE WHEN AND IF WE COULD CLB BACK UP TO FL310. WE ANSWERED AFFIRMATIVELY. IT TOOK 5-7 MINS BEFORE SPD STARTED SLOWLY TO INCREASE. EVENTUALLY WE CLBED BACK TO FL310 AT A RATE OF 200-300 FPM. AT ALT (FL310) ALL ANTI-ICE SYS OFF. AUTOPLT AND THROTTLE ENGAGED. SPD NORMAL; ALL INDICATION NORMAL. CONTINUED FLT WITHOUT FURTHER PROBS. NO TFC CONFLICT DURING THIS SIT. AGAIN; THE ONLY INDICATION OF POSSIBLE ICE WAS LOSS OF AIRSPD.