The increased idle speed could be causing the advance in timing. If you watch the timing mark under a timing light, you'll see it advance as the speed increases. The increased idle speed might be linked to a faulty or sticking idle valve. Easy test would be to simply unplug the electrical connection while at idle, if everything is working correctly, the idle should increase, if the valve is faulty there should be no change (shadetree test mind you).
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.