So I bought a 93 Mercedes 190E 2.6 (V6) for my son as a first car -
safe and affordable.  All seemed well when we first purchased the
vehicle back in November 06.  It drove fine for several weeks
straight, with driving at least some distance at least a few days per
week.  

Beginning in December 06, the car sat for a couple of weeks without
being driven at all, in cold weather outside (some rain, but not too
bad).  Tried to start it up after about 2.5 weeks, and it would try to
turn over but would not start.  My wife got it to start later that
same day, but as soon as it was put in gear, it died.  

Next, the check engine light came on during one short drive.  We
decided to wait and see.  We drove the car infrequently for a week or
two and during one drive, with the check engine light still on, the
car’s head and taillights began blinking while we were driving.  We
got to our destination and it was running rough.  Tried to restart and
at first it started a couple of times but died when put in gear.
Then, it wouldn’t turn over.  The lights blinked even when the key was
out and the engine was off.  

Had the car towed to a mechanic the next day, and of course, as soon
as it was there it started right up.  They said the alarm was disabled
and could not explain the lights blinking.  The main focus, though,
was on the starting issue.  They said the diagnostic machine said the
check engine light was due to the need for an oxygen sensor, so we
replaced that.  The light has remained off since.  They also could not
get the car to not start.  They drove it for a few days and no
problems.  They suggested a likely culprit was the electrical override
relay, so we replaced this too.  

After picking the car up, we have only driven it infrequently for
about 3 weeks.  One time, started it up fine and drove for a few
minutes.  No problem.  Yesterday, about a week later, started the car
fine, drove for about 5 minutes, idling began getting rough, and
parked at the store.  Wouldn’t start after that.  It tried, but
couldn’t quite do it.  This was last night.  This morning, it started
fine.  Drove for about 5 minutes home, then turned it off.  Turned it
on and idled the engine for a couple minutes, then turned it off
again.  Tried to start it again, and same issue.  It wouldn’t start.

I should note that we’ve had the same gas in the car since we filled
it up back in late November - it was 89 octane from Chevron.  

This seems to be an issue with the 190s according to what I read.
Some say it may be the gas, others point to mechanical items.  Several
say replace the relay we’ve now already replaced.  

I’m looking for input and would appreciate any suggestions.  Seems
like when we don’t drive much, this issue tends to happen.  Help!
Thanks.