When I installed the oversized valves I reamed the guides with the proper reamer. I agree with your observation that the valves might be the smoking gun since they were changed during the rebuild. I do not have access to the proper tool to measure the valve guides - a split ball gauge - so I compared the feel of all four valves to the feel of the valves on a sister engine. One felt a little tight, so I helped it a bit with the reamer, and after that it felt OK. Nevertheless, sticky valves could be the bad guy here. I'll have to figure out a way to actually measure the valve guide IDs. Measuring the valve stems is not a problem. Thanks for your input.