'83 300D Turbo

So the gf said that her car will start to slow down from 55 and get slower and slower while driving sometimes and I actually experienced the issue just now. The car would not accelerate at all. I pressed the accelerator and it kicked down but it felt like the car was pushing a brick wall.

At the gas station I checked the brakes for excessive heat and the fronts were normal and the rears were cool.

The clear inline filter showed a little bit of gunk so I'm going to swap that out and hopefully I have a spin on filter and can see what happens. She just filled the tank so I really can't remove the tank strainer.

Any ideas or am I on the right track?