Having erratic idle RPM problems.  When stopped in traffic, for example,
rpm's at idle go to 2000 +, sometimes.  Other times it is normal at 900.
When I shut the engine off at 2000 rpm's and restart shortly thereafter,
rpms are normal.  On other rare occassions, again while in traffic, rpm's
have gone to 3000 at idle.  A couple months ago I advanced the timing, but
don't know if this is the cause.  Any suggestions on how to diagnose or
correct?  Might it be as simple as adjusting the throttle position switch?