SMT FAILED TO MAINTAIN CLRED ALT WHEN ALT HOLD FUNCTION OF AUTOPLT MALFUNCTIONED ON CLBOUT FROM LAS.

Narrative

DEPARTED LAS. RECEIVED INSTRUCTIONS TO CLB TO 7000 FT ON AN ASSIGNED HDG BY LAS DEP CTL. I TURNED TO THE HDG AND SELECTED THE ASSIGNED ALT (7000 FT) IN MY ALT CTLR. I ENGAGED THE HDG BUG -- WITH AUTOPLT 'ON.' ACFT TURNED TO HDG. I WAS CTLING MY RATE OF ASCENT WITH THE ELECTRIC TRIM. AT THIS POINT; WAS ABOUT 1500-1700 FPM. UPON REACHING 7000 FT; I ENGAGED THE ALT HOLD. THE ANNUNCIATOR LIGHT SHOWED 'ENGAGED/CAPTURED.' THE ACFT BEGAN TO LEVEL; THEN UNEXPECTEDLY; THE ACFT PITCHED UP VERY SHARPLY. I IMMEDIATELY GRABBED THE CTL WHEEL AND PUSHED THE WHEEL FORWARD AND DISCONNECTED THE AUTOPLT AT THE SAME TIME. I PULLED BACK PWR AND DID A VERY RAPID DSCNT TO MY ASSIGNED ALT; 7000 FT. I FLEW THE REMAINDER OF THE TRIP WITH AUTOPLT DISENGAGED. I BELIEVE THAT I GAINED ABOUT 700 FT BEFORE I WAS ABLE TO STOP THE DEV. I SPOKE WITH THE DEP CTLR ABOUT THIS. I ALSO CALLED ATC; UPON LNDG AT MY DEST; AND HAD A DISCUSSION WITH DEP CTL VIA TELEPHONE AT LAS. I KNOW THAT THERE WAS TFC AT 8000 FT AT ABOUT MY 8-9 O'CLOCK POS AND AT ABOUT 2 MI WHEN THIS OCCURRED. I WROTE UP THE AUTOPLT ACTION ON THE FLT LOG. OUR MAINT FACILITY WILL LOOK INTO THIS UPON MY RETURN TO PRESCOTT. I HAVE NOT BEEN USING THE AUTOPLT SINCE THIS OCCURRENCE.