A new power loss

[seabrighter]

After a little freeway driving last weekend, I have a fresh concern. 1983 300D, on biodiesel. Fuel filters are fresh, alda line has been cleaned, diesel purge done recently.

When driving on the freeway for an extended period of time (20min +), the car starts to lose power. At first I have good acceleration, getting up to 75 mph without much effort. Once the car has been holding 60 or 65 for a bit, it starts feeling weak. There is a significant power loss, and it gets harder and harder to accelerate. When I get off the freeway, the engine rocks a lot at idle, like a panting dog.

This got so bad the other day that I was having trouble accelerating up to 40mph. I pulled into a gas station, checked the oil. It needed about a quart of oil, and a little fuel, so I added both. The car rested for about 15 minutes.

When I started up again, the acceleration was good again. But, sure enough, after about 15 minutes of driving, it got sluggish again, and was pretty weak by the time I got home.

Normally I drive around town, staying under 45mph, so I don't get to experience this problem. What is this a sign of? Why is it intermittent? All my gauges read normal when this problem occurs.