Friction lever slipping, am I missing something?

Here's an issue with a 105 front shifter, an older downtube bike with a
SIS 7 speed. When in the big ring in the front derailleur, the shifter
will slip and have to be re-trimmed off frequently, I have not tried to
see if it will drop to the small ring, but it will drop the cage down
and rub the chain. It's a fairly NOS 105 shifter. It moves freely, no
matter how much you tighten the center screw. I took it apart and there
are only 4 parts, the shift lever itself, the bushing that fits the
frame, a bushing that fits between the frame bushing and the lever and
the screw. Is is missing something? It's also clean and dry, it would
seem to me that you would want a bit of grease on the metal to metal
surfaces, but I think it would slip even more then. TIA