THE ACFT, A GATES LEAR JET 24B, MODIFIED BY A HOWARD/RAISEBACH MARK II CONVERSION, WAS CLEARED FOR TAKEOFF ON RWY 28L. TOWER PSNL ESTIMATED THAT THE ACFT ROTATED FOR TAKEOFF AT 3200' DOWN THE RWY & WAS AT APRX 200' AGL WHEN THE PLT WAS INSTRUCTED TO CONTACT DEPARTURE CONTROL. WITNESSES OBSERVED THE ACFT CLIMBING IN A WINGS LEVEL ATTITUDE WHEN IT ENTERED THE BROKEN CLD LAYER AT ABOUT 600' AGL. MOMENTS LATER, THE ACFT WAS OBSERVED DESCENDING OUT OF THE CLDS AT A STEEP ANGLE& IN A LEFT WING LOW/NOSE DOWN ATTITUDE. THE ACFT IMPACTED BETWEEN RWYS 28L & 28R, APRX 10,000' FROM THE APCH ENDS. WITNESSES REPORTED THAT THE ENGS SOUNDED LIKE THEY WERE OPERATING AT A HI PWR SETTING. AN EXAM OF THE ENGS REVEALED EVIDENCE THAT BOTH WERE AT OR ABOVE 92% PWR AT IMPACT. NO PREIMPACT PART FAILURE/MALFUNCTION WAS FOUND. RECORDS REVEALED2 AUTOPLT DISCREPANCIES ON 10/4/84; ONE INDCD THE ROLL MODE WAS SENSITIVE; THE OTHER INDCD THE AUTOPLT INTERMITTENTLY ROLLED THE ACFT INTO A STANDARD RATE LEFT BANK WHEN ENGAGED. RPRTDLY, THE AUTOPLT WOULD NOT HAVE BEEN USED FOR TAKEOFF.