Car surges at slight up hill at 50-55 mph? could it be inline fuel filter?

That is possible. only way to find out is put a fuel pressure gauge in line with the pressure line. and when the problem happens, check the pressure of the gauge, if the pressure drops drastically, it is a sign that either you have a restriction, or the pump maybe shorting out under load. causing fuel pressure to drop and vehicle to run lean and stall/cut out.