Need some advice:
Hopefully this version of my question is readable:

Bought the Mercury GM  new and have kept up the maintenance. Got a
nagging problem that seems to be getting worse byt he day. The engine
light keeps coming on and then going off for a while. All fluids and temp
remain normal but have a real erratic idle primarily especially when cold.
6 months ago I took it to a local shop and he cleaned the throttle body
and the surrounding area and I think he replaced the idle air control valve...
kind of worked...no more engine light( until now) but from time to time I
got the erratic idle especially when cold.  The scan tool says P1507 and
as of late P156. P1507 puts me back in the idle control valve area and
throttle body...the P156 says it a oxygen sensor rear passenger....may
be the effect of the P1507 problem. At this point, I just don't know and
was hoping someone out there may have seen this type of problem
before. Also the car has been great and now has 148k on the
speedometer and consistency get 20 miles a gallon. Appreciate any advice

thank you
Lee