Well, there's a limit in the federal tax code; a contribution holiday needs to be taken whenever the accumulated surplus is more than twice the normal annual cost of providing the pension benefits. In the nineties, certainly with high proportions of the assets in equities and good results in the equities, surpluses were being developed and contribution holidays were taken that, in retrospect, might have been better not to take.
On September 28th, 2006. See this statement in context.