i've got a 1983 300D turbo, and i'm wondering how hard i should be pushing this thing. i've reached 95 before, but that was going downhill and i quickly backed off (i was more worried about speeding tickets than i was about the car). what's the highest safe cruising speed for an 18 year old diesel engine like this?

on a longer freeway trip i noticed that if i cruised for a while at 80-85mph, the seatbelt light would come on, the tachometer would freak out (no change in actual RPM's), and then would go back to normal if i slowed down. i asked my mechanic about this and he told me that it might just be the car's subtle way of telling me that it doesn't like to cruise at 85mph. but it will occasionally do the seatbelt light thing at much lower speeds, too.

what could be causing this? is there a relation between the higher speed and the electrical oddities?