Lucas oil treatment is a very good additive that might help. It makes the oil stringy and sticky which could help stick it to the valve stems. To me, it sounds like a oiling problem, and the valves are sticking when hot. The guides maybe on the verge of being to tight, but since it runs so good when cool, I doubt you have any burnt or bent valves. Once bent always bent, same with burnt, you will hear it in the intake or exhaust, these two problems do not go away on there own. Running with out the oil shields and or seals should add more oil to the stems, at 2600 miles, they should be pretty well broke in already, so all you can do is add more lubricant to see what happens.
Try going up one jet size in the carb, a little richer mixture should help cool the valves.
I would bet you are right about the altitude and fuel supply being a big part of your problem. If you had a thermocouple on the exhaust pipe, I would bet you would see some high temperatures right as it starts acting up. Thats how I found a problem with a head I worked over on my 250. Climbing hills under a decent load, it would go lean and really run terrible. With a thermometer and thermocouple on the head pipe, I could see the temp spike and peg the 1000 degree thermometer. Under normal driving, the pipe would run in the 750 to 800 degree range.
Working on the side of the road is not a good idea, but, if you can get puller over and get the valve cover off fast enough, you just might be able to pin point the problem. One or two rocker arms should be very loose, if they all check ok, then the problem lies with a bad lifter pumping up holding the valve off seat. This will be harder to see. It was mentioned before to back the adjustment off to zero lash + 1/4 turn or so. If you can get them to zero lash, the lifter can pump up fully and still not hold the valve open. They might be noisier when cold, but it's a good test to try if you run into a dead in.
Joe
