THE PLT ATTEMPTED LIFTOFF TWO OR THREE TIMES BUT AFTER EACH TRY THE ACFT WOULD SETTLE BACK ONTO THE RWY. WHEN ABOUT 50 YARDS FROM THE END OF THE RWY, THE ACFT AGAIN LIFTED OFF THE GROUND AND CLIMBED TO ABOUT 15 FT. THE PLT STATED THAT THE STALL WARNING ACTIVATED AND HE IMMEDIATELY ABORTED THE TKOF. THE ACFT TOUCHED DOWN NEAR THE RWY END AND THE PLT INTENTIONALLY STEERED OFF THE RT SIDE OF THE RWY IN ORDER TO AVOID A STEEP DROP-OFF INTO A RIVER BED. EXAM OF THE ACFT AND ENGINE DID NOT REVEAL ANY PREIMPACT FAILURES OR MALFUNCTIONS. THE PLT STATED THAT HE DID NOT ASSURE THAT THE MAGNETO SWITCH WAS PROPERLY SEATED IN THE BOTH POSITION PRIOR TO TKOF AND AS A RESULT OF THIS THE ACFT WOULD NOT CLIMB PROPERLY. THE ACFT'S CERTIFICATED GROSS WEIGHT WAS EXCEEDED BY 12 POUNDS AT TKOF AND THE TKOF WAS ATTEMPTED WITH A 5 KNOT TAILWIND.