I don't know how the rockers could get that loose.... I'm sort of with Ron - are you sure that you are preloading them while on the flat part of the cam? If you are preloading the rockers while on the "hump" of the cam, then as it comes around to zero lift you'll end up with a lot of slack.
You could also get a lot of slack if you've bent a pushrod... Take a loose valve and doublecheck that you haven't bent a pushrod?
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Charlie McCall
1985 DeTomaso Pantera GT5-S #9375
"Raising Pantera Awareness across Europe"
http://briefcase.yahoo.com/gt5s_1985
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