Why do I keep breaking rocker arms?

I figured I'd post an update to this debacle. About two weeks after the above post I had to take Val back into the shop as she wouldn't keep running, which turned out to be a loose wire (I've just got to learn something about classic automotive mechanics!!). In the process of diagnosing and fixing that ten minute job the machanic noticed that the engine 'didn't sound quite right' so he pulled the valve cover and -surprise!- a broken rocker. I had been regularly driving, no hot rodding or anything. So I left the car for them to figure out the issue.

I just got it back yesterday. They tried everything they could think of and finally just pulled the engine and tore it apart. It turned out that the idiot subcontractor in Texas that did the rebuild had used mismatched lifters on two of the cylinders -the two that kept breaking rockers. So now I have a new set of matching stock lifters, new aluminum roller rockers, a replaced oil pump (they found a crack) and a new balancer. All under warranty. Part of me feels bad for the shop, part of me feels like they deserve it for subbing out work. I'll still be looking for a new mechanic in the area. I mean, it took four months.

The good news besides the problem being (hopefully) solved is that the car runs and sounds like a dream. The loud clattering noise is replaced by a much quieter sort of buzzing sound and I can actually hear my cool exhaust system now, and my stereo as well as long as I'm driving under eighty or so. Much better acceleration especially from a dead stop and on hills as well. So I guess Santa is real after all.