Any one know what might cause my P/A 40 UL on a pipe to start to quit off an on at the end of a flight, this just started lately, before it would just lean out a  ???bit then a clean stop, nothing has change on fuel or prop?

Jim the tank is in tight inside of fusalage. the flight run is right on, the pipe is  the same.

Check for a leak.  It don't take much.  I had one leak which wound up being a take apart filter.  With silicone tubing I have even had new tubing leak.

Randy surely knows his engine business, I dont question him, but I have had a similar situation when I changed the trim on one of mine, I have to second Alan's question, have you changed the leadout position? If the plane is flying in a slightly different Yaw position, it can change the fuel pickup characteristics.
But as Randy said, check all your fuel system as a baseline ,,

Thanks guys I will start at the fuel system as Randy said and go from there.

Any one know what might cause my P/A 40 UL on a pipe to start to quit off an on at the end of a flight, this just started lately, before it would just lean out a  ???bit then a clean stop, nothing has change on fuel or prop?

Your second post added that you see absolutely no other problems. That would make me lean more towards the Mark S. and Steve H. thinking. Any tank issue other that a subtle yaw issue would almost certainly show up during the run, not just in the last few drops in the tank.

The fine line on model yaw, tank skew, is discussed in some depth here.

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=257774&mesg_id=257774&listing_type=search#257909

One lesson here is a fraction of a degree can cross the line where residual fuel remains in the front of the tank instead of the rear of the tank, causing the three-lap-burp-dance. The change can be small enough you didn't knowingly change anything in trim -- weight gain from fuel, unnoticeable warp that causes no other problem.

Many planes do end the run with a little residual fuel remaining in the front of the tank, and the flyer is never the wiser -- until minor yaw creep moves it closer to the line, creating a burp dance from the other side.

     Larry Fulwider

If the fuselage is wide enough cant the rear of the tank out about 1/8 inch. Have a couple four strokes that cutoff is scary clean. No warning at all they just shut off like a switch..