I've got an 86 Westy, that recently started losing power at highway speeds.
It dosen't backfire or buck, just loses power gradually. The van has the
wiring harness "kit" already installed.Though this problem is unlike the
"harness" problem. It seems only to happen when the van is up to temp. and
under load, and shutting off the key provides a brief 2 or 3 minute respite.
If the van sits and cools down for awhile it can run fine for maybe for a
week or an hour. ! Dry or wet weather makes no difference. I searched thru
the archives but it seems everyone has a different answer. You cna respond
to me directly or to the list.  Thanks, Kris Weber

P.S.  I've been off the list for awhile, though I did manage to rack up
about 12,500 miles on my Westy over the last 5 months. {with no problems} .