NEW YORK STORY

Just when summer music promoters were as happy as Deadheads in a rain shower, a rash of ailments derailed two of the year's most successful tours. The Guns N' Roses/Metallica tour shut down for more than two weeks after an Aug. 8 Walpurgisnacht in Montreal: An exploding flash pot left Metallica singer- guitarist James Hetfield with first-degree burns on his right arm, ending the set suddenly, and after Axl Rose walked off stage with what were later called throat problems, a riot ensued. The tour will finally resume in Phoenix on Aug. 25, and if Hetfield isn't up to it, the group will temporarily feature Metal Church's John Marshall. The mayhem might confirm Axl's rumored fear of cities that start with M, a superstition fed by his psychic. A show in Minneapolis has been canceled but rescheduled for Sept. 15. The tour was to end on Sept. 5, but now, says a GN'R % spokeswoman, ''They're having to reschedule around football season.'' Meanwhile, the Grateful Dead have nixed their fall tour while Jerry Garcia recuperates. ''He has lung disease,'' says a Dead spokesman. ''Thirty years of Camel Straights will leave their mark.''