Several years ago I had the pleasure of rebuilding my wife's 86 with Robert Lilley, who replied to a query I had on swapping engines.   My wife's van has run so well I have been off the list and out of the VW repair business for quite a while.  I had the great idea to buy another vanagon 89, and yes I am back in the repair biz.  Heres the problem ...  The first 300 miles it ran great, very cool, well below the half way mark on the gauge.   Then it started getting hot, then the oil buzzer came on.   I have traced the oil buzzer down to  the high setting.  Coming off the highway when the RPM's drop below 2500 it goes off but will stop if you tap the accelerator above the 2500 mark, like it gets stuck momentarily .  Changed the oil to 20/50 put on a mann filter, still runs hot.  Changed the thermostat, both fuse relays and 70amp lead strip.  Droped the radiator and had it flush tested???  Not sure the competency level was there, but the tester said it seemed restricted, no exact science.  Changed the fuel filter, awaiting a new fuel pump arrival.  Any suggestions on what to check first on the heating up problem, and also the oil buzzer.  
With 4 kids 17 on down my wife will not consider driving anything else, therefore I am the VANFANMAN by default, when mommas happy ...
THANKS