Annoying Issue after driving on the highway for a while..

[Tepot]

maybe I'm wrong. but if you eliminate the oil problem. Have you check your timing? I've recently some how got the same problem. I don't have a gauge for warm oil temperture. but I think your car is With electronic fuel injection. might be different. I have a carb on my 74. Mine was accelerating good but not like it was supposed to. especially around 70 MPH and up. And turned out that not only my timing was way off but also that my vacuum advance was connected to wrong vaccum port on carb. So I might be very wrong on this but could be timing or vaccum advance.

I might be wrong but maybe this can help. I know my problem is solved now and sounded pretty much like yours.