Falling off the reg means the main cylinder pressure has gone below the reg pressure so you're getting air at the same pressure either side of the reg i.e. the pressure has dropped below 95 bar and working as if the there is no reg there.

flat string = a string of similar velocities like you have been getting but that doesn't spike at the end just drops down.

Missed the fact it should be 95bar which is on the low side anyway for a .177, ignore me saying turn it down you can also eliminate the spike with the other settings, to save a load of typing and will be a lot clearer watch Rob Lane's video on what to do.https://www.youtube.com/watch?v=4gBFdY3YDxc