Both a conversation with the guys at quadrocopter.us, and now Ziggy confirm that something about the Heavy Lifters is causing the caps to vibrate and snap off. The recommended "fix" is to hot glue the caps down.
I stare at my caps and try to imagine the vibration it would take to make them snap off, and then I drift off to a place where all my brain cells shut down. Then, I remember the History Channel show I saw about Nicola Tesla, who invented some sort of resonance machine (Earthquake Machine) with which he claimed he could bring down buildings. The story (folklore) was told of how he attached his machine to a half-built steel building:
"He put his little vibrator in his coat-pocket and went out to hunt a half-erected steel building. Down in the Wall Street district, he found one: ten stories of steel framework without a brick or a stone laid around it. He clamped the vibrator to one of the beams, and fussed with the adjustment until he got it.
Tesla said finally the structure began to creak and weave and the steel-workers came to the ground panic-stricken, believing that there had been an earthquake. Police were called out. Tesla put the vibrator in his pocket and went away. Ten minutes more and he could have laid the building in the street. And, with the same vibrator he could have dropped the Brooklyn Bridge into the East River in less than an hour."
I digress. The fact that this vibration can snap off the caps is fascinating and alarming at the same time.