Noooo. Insulating under the roof decking will do nothing except decrease the life of your roof.
The best thing to do would be to install a solar powered roof vent fan that operates on a thermostat. Place the power vent mid way between the two gable vents.
For more information:
Read "Should we use open cell or closed cell SPF insulation on the underside of our attic roof in southeast Louisiana?" a Q&A answered by Rick Goyette.