I was reading a book in the library the other day on maintaining a Volvo 240 and came across something interesting and might explain the strange incidents of retarded timing on a decent running, but older engine.

It stated that valve lash use to be set at .018 inches, but when knock sensors were introduced, they had to change the specs to no more than .016. The reason for this is the knock sensor interpreted the valve noise at .018 as ping or knock and would retard the timing up to about 8 degrees I think it was said.

So I think when I get a chance, I will go check the valve lash. I plan on setting it around .014.