Question along those lines - for a SHORT while I was into boating, and was talking to some diesel guys. They do this thing called "tested log" racing - you race against your prediction of your own time and try to come as close as possible to it, rather than first-to-the-finish. Closest prediction wins. Sort of interesting because they have a monitor come aboard and verify that you aren't using any sort of speed gauges - you can have RPM, but nothing that tells you how you're doing time-wise.
Anyway... I was watching this guy drive and every few minutes (like 5-10 minutes?) he would throttle up or down a few hundred rpm, then go at that rate for a while, then change again. He said it was because "diesels don't like to stay at one speed for long."
Now, I was kinda young at the time and naive - didn't want to admit I had no idea what he was talking about so I just accepted it and it stuck with me. But now I'm wondering - is there any truth to this? I know gas engines way better than diesels - in a diesel, is this true, and why would it be?