Why the change in RPM and Mileage
 

Since I bought my 2001 Beetle Diesel, I've experienced a phenomena that has me puzzled. Upon taking it out on the highway after it sat for several days, the vehicle idled at less than 500 rpm. And, at 70 mph, it was idling at under 2000 rpm. Normally, it idles at around 1000 rpm plus and tacks close to 2300 rpm when going 70. When this happens, I instantly note improved mileage (once getting over 60 miles per gallon compared to the normal 42 to 45). However, when I stop my car and restart it, it goes back to normal. Why? What's happening? And, is there a way to always get it to run at the lower rpm on the highway (some kind of chip or computer remapping procedure).