Not to panic but when you say your tank is low, it makes me nervous! Are you up on End of Tank Dump? EOTD can come around when the liquid is gone from the bottle and the pressure begins to drop. a weird thing with regs is that can cause the remaining gas in the bottle to go into the tank in a "dump" which can be very bad for any fish.
The bubble rate changing may be because the needle valve is not quite as good as some. Not up on the diffuser end though so I'll let others have a go at that.