THE AIRPLANE DEPARTED JONES (RIVERSIDE) AIRPORT AND CLIMBED, ON COURSE, TO ITS ASSIGNED ALTITUDE. IT WAS IN IMC CONDITIONS AND A IFR FLIGHT PLAN HAD BEEN FILED. THE PILOT TOLD ATC (AFTER BEING ASKED) THAT HIS RIDE WAS 'SMOOTH SO FAR'. SHORTLY THEREAFTER THE AIRCRAFT STARTED VEERING RIGHT OF ITS ASSIGNED HEADING. THE PILOT WAS ADVISED OF THIS AND ACKNOWLEDGED. A CORRECTION WAS INITIATED BUT NOT COMPLETED. THE AIRCRAFT AGAIN VEERED RIGHT, ENTERED A STEEP DESCENT, AND DISAPPEARED FROM RADAR. THE PILOT DID NOT RESPOND TO FURTHER RADIO CALLS FROM ATC. THE AIRCRAFT CRASHED IN A STEEP NOSE DOWN ATTITUDE WHILE AT A HIGH RATE OF SPEED DURING A HEAVY RAINSTORM. POST ACCIDENT EXAMINATION REVEALED NO EVIDENCE OF PREIMPACT MECHANICAL FAILURE OR MALFUNCTION OF THE AIRCRAFT. PATHOLOGICAL AND TOXICOLOGICAL EXAMINATIONS OF THE PILOT REVEALED NO EVIDENCE OF A HUMAN FACTORS PROBLEM.