Hi, everyone -
Once again, I could use some advice.  I have a plain old '85 Vanagon.  

Here is some background:  On Thursday, after work, I started it up and it did
not want to idle - stalling unless I kept my foot on the gas.  I drove around
in circles in the parking lot for a little while, and when it warmed up, it
ran and idled ok for the rest of the trip home, but it has been doing this
every time I've started it since then.  This morning, I had an oxygen sensor
installed - the last thing I needed to pass emmisions inspection - and I also
had them change the fuel filter.  

On the way home from picking up the van, the temp. light turned red and the
temp needle pegged all the way to the right.  It was that way for about a
mile, then everything went back to normal.  I got home, topped off the
coolant, and noticed the oil filler tube and cap were missing.  The van still
does not want to idle when I first start it up, and it is still doing the
sporadic overheating thing.

Ok, so here are my questions:
      Can any of these problems be related to each other?
      I have ordered a new fill tube and cap, but in the meantime, should I
put something over the hole?  What?
      Should I suspect the thermostat with this sporadic overheating
problem?  
      Is it normal for the oil filler tube to simply fall off?

Thanks in advance for your help!
-Lauren