Well, after all that electronic testin, cussin' & discussin' it looks like the problem is a mechanical one,ie the points were "set" too wide. I dicovered this by tweaking them open too far, which induced the same problem only all the time. So I backed off the gap a little at a time untill I had too little, then opened them up just a hair. I'll have to try the trick with the cardboard to get them a little closer to spec. My guess is that the gap was just barely wide enough & when it got hot enough the plate side of the points expanded enough to open the points all the time. I'll have to take a longer drive in hot weather to be sure, but I think I've got a better handle on the problem now. Kudos to all who contributed ideas. It's great to have such a resource to rely on. Thanks again!

Mike M.