I am reading up on this and read it could be air in the system. The recommendation is bleed or "burp" the cooling system. The factory service manual I found on-line has detailed instructions on how to do this. I'll give that a try this weekend. Might replace the belt too, while I'm at it.
I'm with Dexter II on this one--a vacuum leak. The reason being is that you state that you have heat at 2000 RPM and lose it at 800 RPM. The lower the RPM the higher the vacuum. When you slow down to the 800 RPM area there is more vacuum and this will cause the 'air flow control panels" (doors) to move with more force. Depending on your vehicle a door is either closing or opening at the 800 RPM range and changing the air flow. A vacuum check on the entire vacuum system is at hand.
From what I understand, the 95 model Villager's blend doors aren't vacuum controlled. I put some coolant in the recovery tank and heating improved. Still checking it out. I found the engine air bleed bolt and heat pipe bleed cap and read up on bleeding the system. I'll try it soon.
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