Thank you for your post, I am sorry to hear that you have been seeing problems.
When you plug the serial adaptor cable back in, are you doing so with the PT104 connected to it?
If so, I recommend disconnecting the unit from the adaptor and reconnecting just the adaptor to the computer. This is because sometimes the computer looks "through" the adaptor and sees what's on the end of it - it then treats it as a different device.
If it still will not work then check again in the device manager - does it show any errors?
Let me know how you get on.