Hi Zhang Jun,
Yes, that did make some difference but in the end that was not the reason for all the problems.
The final solution turned out to be really silly.
I was not using the FRDM-KL03Z board on its own but I was also using a digital scope to check my incoming signals.
It turned out that the scope was not plugged into the mains with an earthed mains plug.
Instead, a modified mains plug was used: one without an earth connection so that the scope could be used for measurements where an earthed mains plug caused problems.
Of course, since then nobody remembered the specially modified plug so when when I started using the scope I did not know either.
I finally found out when I started getting electrical shocks from the scope and the connected freedom board.
Because of the missing earth connection and some filtering capacitors sitting between the mains supply input and the earth connection in the scope there was actually +/- 120 V AC ( half the mains supply ) on the scope's ground.
The filter capacitors don't allow much current so it was not destructive but it sure wrecked havoc on the USB link to the Freedom board.
Connecting the PC's USB ground and that of the freedom board to the scope's ground with the 120 VAC on it is definitely not a good idea !!!