Thanks for the tip.  Is it wierd that I'm enjoying this troubleshooting exercise this much?  Pity about the transient nature of the running problem tho.
Rollovers and gaskets are new as of about 2 years ago. 
 Thinking about it, and the nature of the debris in the tank (incl a chunk of steel), I suspect that I may have introduced it during the installation of the tank. I installed the tank without the sender, so it is possible debris was introduced while wedging that sucker in there. I did break the hose nipple on one of the rollover valves during one tank install attempt, and replaced that one, so it is possible I broke it again and didn't notice...so that is a possibility as well. 
 I think will clean the tank, and check it again after a few months of driving. I'll also try and view the bottom of the rollover valves, in case there is something obvious. I may attempt an overfill with the charcoal canister disconneted to see if the rollovers leak. 
 Then back to the running issue troubleshooting.