Hello.Maybe you have a clogged up catalytic converter. I had one
once,though not in my wonderful vanagon.This clog can cause low power and
overheating.Low power that you can tell even in any vanagon.Dont know about
the white smoke.

Are you loosing coolant? Do you suspect a  coolant loss into a cylinder?
Maybe try this.Warm the engine up till it is about to overheat.Disconnect a
spark plug wire and see if the engine  continure to get  warm till it does
overheats.If it does, you may have found the bad cylinder. If not, move
onto the next cylinder.

Some times just removing one sparkplug wire at a  time is enough to tell if
you have a leak.See if the foaming stops when a wire is disconnected.This
is for a bad leak.

Here is a test to check for carbon monoxide in the cooling system.Kinda
like the wand that the emmision people have. Get a carbon monoxide detecter
from Walmart. Has a digital readout that goes from 0-999PPM.$30, and looks
like a smoke detector(110Volt).Remove the cap from the expansion tank.
Next, get a 1 gallon milk jug and cut the bottom out 3/4 the way around.
Duct tape this jug to the expansion tank,and put the detector inside.Hook
up an extention cord and plug it into the wall. Tape the jug botton closed,
but not a perfect seal.Wait for a few minutes till the exhaust does its
stuff ,or not. A loud warning will sound or the ppm meter will register. It
takes a few minutes, and you need to follow the directions for the
detector.Dont tell the manufacturer what you are using it for.