power loss uphill

[tbone90]

I purchased a 1984 300D with 190,000 miles this month. Yesterday my father was driving the car in a hilly region and experienced serious slowdown from 50 mph to 20 mph going uphill with rpm going to 3000. When he stopped at a red light the car went dead. This happened twice it restarted after a few attempts.
This sounded like a fuel system problem ,from my limited knowledge my thought was to change both fuel filters. From the previous owners records this maintenance does need to be performed.

My father initially thought the transmission might be to fault. It does shift hard.
My questions is what is the problem? and what action should I take?