Anyone know Briggs&Stratton motors?

Ok if someone can help i would appreciate it.I have an 18H.P. lawn tractor with a Briggs&Stratton motor it will work fine for the longest time then all of the sudden it will start missing like hell and blow large amounts of white smoke out the exhaust.I then disengage the mower and idle it down and it clears up,then maybe another 20-30 minutes later it will do it again.I called there hotline what a joke that turned into all prerecorded crap,i was on for 20 minutes and never did talk to an actual person :x Also tryed Sears where i bought it and the same thing.Why can't they have atleast one live person to talk to :?: Any help would be appreciated. :D