2000 xcursion miss firing but stops after another key cycle

[7.3sandtree's]

Ok so I took my excursion to the shop and had diagnostics done. They tell me it throwing 3 codes; one for the glow plug circuit, back pressure circuit and one for the #2 injector. when I drive it will be fine one minute and miss firing the next; when I pull over and do another key cycle it will run fine again and then start miss firing again.
I have heard that the clips for the injectors and wear out and cause the injector to be loose. My question for any one who can answer. Will a loose injector cause the truck to throw a glow plug circuit code and a back pressure circuit code? The mechanic wants to replace my ecu and I don't want to pay 1600 bucks in labor and parts for a loose injector.