I have a volvo 740 sedan (152000 mi) that hesitates/sputters, especially
when cold and taking of from standing still and ocationally/randomly at
other times. It also runs a little rough when driving at constant speed.
This started after the car broke down (due to stalling and sputtering) and
I had to replace the oxygen mirror. I had a complete tune up done and I
also had the mechanic double check the oxygen mirror and everything seems
ok. I also had them check the fuel pump/system and it works good. I did a
injector fuel cleaning at mechanic and after/during that the car ran
good/normal. After driving up a tank the problem reoccured.

Things I know can effect this are:
I have some soot on the engine, at the manifold gasket, at one "cylinder"
that appeared during the breakdown and there might be air leaking in
through there. Also I drive it on regular and I have not yet tried to
drive it on any other fuel grade. However,I do not believe that it would
make a big difference.

I would like to know what else it could be.

Also I am considering doing a 150000 mi service which, from what I
understand, includes changing the timing belt. It has been done once
before at 50000 mi according to previous owner and I was wondering if it
is nessesary/worth the money to do it again at this high milage (152000).

I am very greatfull for any help.