DURING AN IFR FLT AT 12,000 FT THE PLT CONTACTED ATC & REQUESTED A HIGHER ALTITUDE TO GET OUT OF IMC. HE WAS CLEARED TO CLIMB & MAINTAIN 14,000 FT. THE PLT ACKNOWLEDGED THE CLEARANCE AT 1855 & WAS NOT HEARD FROM AGAIN. RADAR CONTACT WAS LOST AT 1859. APPROXIMATELY 1 HR EARLIER DURING THE FLT THE PLT WAS ADVISED OF A SIGMET THAT INCLUDED MODERATE TO SEVERE TURBULENCE BELOW 18,000 FT WITH STRONG UPDRAFTS & DOWNDRAFTS. EXAMINATION OF ATC'S ITAP REVEALED THAT THE ACFT'S GROUND SPEED DECREASED FROM 110 TO 70K AT APPROXIMATELY THE TIME THE PLT RECEIVED HIS CLEARANCE TO CLIMB TO 14,000 FT. SHORTLY THEREAFTER GROUND SPEED INCREASED TO 90K. IN ADDITION TO THE GROUND SPEED DEVIATION FOLLOWING THE PLT'S ACKNOWLEDGEMENT OF THE ALTITUDE CHANGE, THE ACFT CHANGED HEADING ABOUT 90 DEG TO THE RIGHT OF THE ORIGINAL WESTERLY COURSE. THE ACFT CONTINUED ON THIS NORTHERLY COURSE UNTIL RADAR CONTACT WAS LOST. THE PLT DID NOT REPORT ANY PROBLEMS TO ARTCC DURING THE TIME OF THE COURSE CHANGE OR GROUND SPEED DEVIATION.