All right. So it's been a while, but I finally solved it. First of all, messing around with all that vegetable oil garbage killed the engine. Please don't ever do it. Whatever, though. As far as the overheating issues... I actually put another engine with about 100k miles in the wagon. It is absolutely amazing, however, it started doing the same overheating under load or uphills again. Changed out the radiator, still does it. Finally, went and changed the viscous clutch, which if you read above, I had a "mercedes mechanic" do and was new. And voila, that was the problem. I no longer have any overheating issues. The viscous the previous mechanic put in was an aftermarket. Lesson learned... Aftermarket fan clutches do not work. I put a mercedes original fan clutch in and it instantly fixed my problem.