A problem with an L.... No way, lol..
I had your same problem with my L when I first bought it. Sent it back and they replaced the piston, liner, regulator diapragm, and gaskets. I got it back, flew great for 20 flights, then started having more problems. I had replaced all I could replace. Even sent it back in. And only ever got about 10 more flights on it.
Anyway, back to your problem I think it's probably an air leak. Remember even though your plane may be horizontal in the air, or sitting on the ground, there is alot of air flow over your engine while in the air. Therefore creating problems such as the one you say you have.
Easiest part to replace also. The valve cover gasket...
Before putting the valve cover back on. I make sure it is 100% flat also. If not I put a piece of sand paper (fine) on a super flat surface, then put the valve cover down on it and move it back and forth slowly.

Also, as it did sit for along time I would definitely replace the diapragm, plunger, and gasket..
Also adjust valves.

I used the dz header with two copper gaskets mounted to the engine. And I can't remember what pipe.