I hope this helps with your engine braking problem.
As Capt. Mike said, there is a deceleration valve (I believe yours is electronic). Check it carefully, along with the distributor advance.
It sounds like your cylinders are being flooded slightly, and when you accelerate the extra fuel burns off as if it is flooded. I would check that out ASAP, as many an engine has had gone to it's eternal rest due to the fact that there is more fuel in the combustion chamber than sufficient oil viscosity.
You mentioned the thermostat -- is the engine running hot down these grades? If it is, most likely you are injecting too much fuel - and the lack of lubrication (due to the fuel breaking down the oil slightly, will cause friction and give you a high-temp reading. Also, check your oil level -- down a grade it tends to gather near cyl. # 1 and #3.
Plus, check the following:
- Vacuum lines, often the culprit, especially if you are running hot - they crack.
- Depending on where you are in CO, make sure that your ignition is timed for your altitude. (don't depend on auto-altitude correction, it blows).
- Learn everything you didn't want to know plus more about your deceleration valve. That is your engine braking buddy, get intimate with it - start with a drink, maybe dinner first. This needs to be on top of it's game.
- Capt. Mike has a great point regarding fuel - make sure that you have some punch in the octance arena, you'll need it to burn off the extra "down the hill to grandma's house" fuel.
THE TEST: When you are approaching the end of a long downhill run, but are not needing to brake yet (give yourself about 5-8 seconds of coasting) - press in your clutch, if auto, put it in Neutral. If the engine stalls or the RPMs drop to near stalling, back to drawing board.
If there is little to no change in RPM's, congratulations you probably fixed it.
I am fairly knowledgeable about Fuel Inj. - but not oil. I don't want to give you the wrong advice here so someone may want to chime in -
If this VW is a hill-monster, up and down, I do know as a fact that more fuel than normal goes into the combustion chamber.
What I'm not too sure about is putting thicker-than-normal oil in the crankcase, to reduce wear as a result of fuel richness. I don't want to say "do it" as I like to make sure that I know what I'm talking about prior to giving any assistance. So, either ask around, or maybe the Capt. will come to the rescue. He got me out of some pretty sticky situations!
'78 SB Conv
'74 SB Auto-stick