I understand your pain.
I am going to assume that each time the cap got blown out that you cut a little bit lower on the vertical pipe to get a fresh seal. If you did not make a fresh cut, you may have an issue.
You should make a fresh cut and don't sand the pipe like you mentioned. Use some primer and Christies Glue. Glue both the inside of the cap and the outside of the pipe, and Twist the cap forcing it downward. You may hold it in place downward for 30 seconds (sometimes the fitting will float up or out a bit).
Wipe up any excess glue.
Let it dry for atleast an hour. When you turn on the water do it slowly don't turn it on fast at all. Slooowly and leave a hose bib open. After the water is on slowly close the hose bib. Slowly.
Good Luck.
A side note: Start this in the morning you will sleep better.
goodluck