Various endcap problems:
Don't use em: You get all sorts of bug frass in the elements that ultimately
clog em up and they get full with water. Also they smell evil and are a mess
to clean up if you ever rebuild the antenna. You also can get whistling in the
Use em: They either pop off due to pressure changes and buildup inside the
element, or they degrade and crack off due to uv exposure.
My current favorite is to use aluminum window screen formed around a slightly
smaller piece of tubing, then shoved into the element or boom ends. I fasten
it with a dab of silicone sealant. The element can breathe, they don't seem to
fill with water and the bugs don't get in there.
Also, re rope. If your elements are fairly tapered, they don't seem to vibrate
too much. Not a physicist, but maybe something to do with not resonating??