I picked up this car as a gift, and on day one drove it the 370+ miles home. After the first 160 miles or so I noticed that the car was struggling to maintain highway speeds. So I pulled over into a rest stop, and took a short break. When I hit the highway again, it was back to normal performance. However after another stretch of driving (160 miles or so), I noticed it was again struggling to maintain 70mph. This time I pulled off the highway, turned the car off, then immediately back on and performance was back to normal.
It's my first diesel, and the first time driving this vehicle.
The car's got plenty of "quirks" but ran (mostly) fine other than this drop in power over time. Not really an issue for day to day driving, but something I'd like to address at some point. Hopefully it isn't indicative of impending near term doom.
The valves have been adjusted recently, and the maintenance seems mostly up to date. I'm not real familiar with diesel's in general, and so as an addendum here I would ask for any information that would allow me to utilize the manuals for keeping this old girl running.
Edit: Additional car specs.
300D Turbo Diesel
According to the Wikipedia article, the engine is: 3.0 L OM617 A I5 turbodiesel