In my case the fuel was literally boiling. That's why I wanted to make sure your fuel pump was cold to the touch. It is not unusual for liquids to boil in a dead headed pump.
I don't suspect a vapor lock condition cruising at 40 mph.
In your case the air could be peculating around/thru a leaky pump diaphragm.
Also your pump check valves could be leaking thus not holding full pressure.
What is aprox diameter of the bubbles? Got any pics of your ride?
Originally Posted by 001mustang
That being said, you did not mention the possibility of a leaky diaphragm.
When you assembled the fuel pump was the pump spring in the correct tension state while installing the diaphragm? Was the diaphragm new or old stock?