GMC Sierra irradic running

I have a 1995 GMC Sierra 3/4 ton with a 6.5 turbo diesel. Long story short, when it first cranks it runs fine for a couple of miles, then it starts to feel like it wants to shut off - sorta like it is not getting enough fuel. I can usually drive along about 45 mph with out much trouble, but if I try to get it on up to 50+ mph it gets really bad. We have changed the fuel filter and put on a new oil pressure switch. This particular truck is know for fuel injector problems, GM even extended the warranty on them and my truck has had 4 or 5 new ones. It was suggested to me that it might be the PDM module on the side of the injector instead of the whole injector. In checking $$ on the parts the is just a couple of hundred $ difference in the price of the module & the whole injector. Is this a good place to start looking into the problem or am I way off base? If I am on the right track, will it be about the same task to change the module as the injector? I mean if I have to take a lot of parts off to get to the module, then put them back and it isn’t the module and I have to redo the whole process to replace the injector, wouldn’t I be better off to just replace the whole thing since the injector comes with the module? Or do I need to check something else completely? PLEASE HELP I have to have this truck to get back and forth to work, no $$ to buy newer one.

You are in over your head, take the car to someone who knows what they are doing with a diesel and use a credit card.

Thank you for input. Do you have a credit card I could use? I do have some folks with some diesel mechanical background helping me.