Really tighten the belt...I mean you can flex it a good quarter inch after it feels tight. Use a big pry bar and a friend to tighten it.

If it still comes on after that check the connection on back of gauge cluster...a wire might be loose. Does it come on at exact same RPM every time you hit that rshared_pm...as in can you slowly raise rpms to that rshared_pm and get light to flick on at same spot every time?