Arduino for STM32

Hi all
I have purchased an STM32F103C8, and am having some problems.
I purchased this of ebay, and I assume its a Chinese clone , without bootloader. I can install the arduino bootloader via stmcube and I return the boot jumper to its origional position and hit reset before removing power. I can then load a program using Arduino via usb. But immediately when the program finishes loading (succesfully) a message comes up and says there is no longer a usb connection, it appears the bootloader has been erased. I can reload the bootloader, but unless I do a full erase beforehand Arduino doesnt see any com port.
I hope someone can shed some light on this problem.
Regards Larry

You need to enable USB CDC in your sketch, otherwise COM port won't be enabled.
If you don't enable USB CDC you can enter bootloader by setting boot1 jumper to 1 position and resetting board (if you flashed HID bootloader).

I'll tell you my experience.
I had such an experience with the 'C6' chip.