I thought I posted this yesterday - oh well ...

I think this is a pressure issue:

1. Water pump seal -check for seepage at the weep hole
2. Reservoir leak (above the coolant level) -usually in the area of the neck (the cap could be OK, but still not hold pressure due to another leak)?
3. Head gasket leak - worst case, but could account for the spikes in temperature you are experiencing

Fix cheapest first - 1 and 2, above, are easy and inexpensive. 3 is expensive, but with megamiles, possible.

__________________
George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg