Gas - Port engine loses 300 rpm in 10 mins.

While underway and throttles untouched, my port engine loses about 200-300 rpms and I have to push the port throttle further to keep it in the same rpm with the SB engine. Any ideas?

The distributor is original with new contacts,
the ignition coils on both engines were replaced 2 years ago.

Sounds like throttle creep.

yes, likely a throttle control slipping...
engine runs smoothly when this happens??? ...assuming both engines turn up proper WOT RPM....