I would think that if you had solids lifters you would have a set of bent pushrods,and/or pulled rocker studs by now. I would suggest backing off the lash by a 1/4 turn each and see if things get better. If you get to the point of clatter,you might just have way too much cam. You didn't mention exhaust, I hope you have headers and low restriction mufflers. You mentioned fattening up the idle mixture, try jumping up the jet size and see if it helps. Hank