My 87 Vanagon will run fine for about 20 mins and then it starts to misfire and
lose power ... especially noticeable on the hills.  The misfiring seems to then
become intermittent.  The misfiring causes the engine to vibrate as if its out
of balance or one cylinder doesn't fire.  It's sputter as if it wants to run
smooth.  If I ease back on the gas, it sometimes will cause it to snap out of
it and run smooth, but usually I'm climbing and as I give it more gas, it'll
misfire again. It is not directly correlated to operating temp, as the temp
seemed to stabilize at normal before the misfiring began.  It started a few
days ago and I opened up the distributor and cleaned up significant deposit
from the rotor and cap arc points.  I thought for sure that that would fix it,
but it didn't.  I also pulled the spark plugs and cleaned and gapped them.
They looked fine ... a bit of brown but otherwise seemed ok (running hot?).  I
put in some techron and am running this tank down in case there's some water in
the gas, but i doubt this will help.

If anyone has experienced this and can point me to some likely culprit, I'd
appreciate it.  Other data points: 137K , new heads about 4K ago.  When it runs
well, it really runs very well.  The O2 sensor light came on about 15K ago and
I never replaced it.  Thanks for any help.

Tom Ervolina
Enterprise Solutions Research
IBM
Yorktown Heights, NY
ervolina@us.ibm.com