New Mystery on my 124 91 300D 2.5

[oldiesel]

Today running on the hwy at about 65mph my engine started missing slightly and intermittently,when i got in to town and stopped for a red light it was very rough at idle.Once i got into a parking lot and raised the hood With the engine running the little clear fuel filter was just full of foam and bubbles.This car is new to me and this is the first tank of fuel,the gauge was at 1/4 so i figured the gauge was incorrect and i was out of fuel,however it only took 14gal to fill it right to the lip of the fill pipe so i guess the gauge is right.
Does anybody have an idea of whats going on here? Is it likely that the lower part of the fuel filter is so plugged to cause such a problem when the fuel gets down to the last few gallons?
I cant do much about it now as the tank is full but i sure would appreciate any advice you might have.Car runs fine once i put fuel in it,actually runs too fine as today i had a manifold pressure gauge on it for the first time and it can easily exceed 22psi but thats a story for another post when i get time to really get into it. TIA Don