Had the exact same issue last year with my 3.5. Oil light would come on when stopping at red lights, but not all the time. In my case, I had a small oil leak coming a hairline crack in the oil pump where the oil pressure switch screws in.
Check the front of the block for oil leakage around the oil pressure switch. If you have oil around the switch, pull the plug from the switch and clean the contacts. If the leakage persists, clean around the switch and check for hairline cracks where the switch screws in. If you can't find any cracks, go ahead a replace the oil pressure switch (fairly cheap part, around 10-15 bucks). ** MAKE SURE YOU DO NOT TORQUE THE SWITCH DOWN **
Above is a picture of mine after attempting to put in a new pressure sensor without checking for cracks prior and severely angering the oil pump. Putting in the new switch (finger tight + two full turns, probably one too many) turned a small dribble leak into a high pressure stream of oil shooting from the block and making the $10 part into a $2600 repair.