Have you used or installed a cam thrust plate? The problem you described may be a cam "walking" problem. I believe this is also ties to (at least in part) to the cam/distrib. wear problem. The best one I have found is at :
www.bluethunderauto.com
I have never had a lifter related problem where I have had to check the bore angles. Performace engines with racing cams, esp. rebuilds that have lots of miles, need this backup plate for the cam.
Tom - Good site at Crane Cams.
cobrashock