Same thing Happened to me once in the Old 90.. Suddenly running like it was on 3 cylinders. Turned out to be air getting in a corroded fuel pipe along the Chassis, Try Priming the fuel filter plunger next time you start it till it's Hard, Make sure you getting fuel up ( No air in system) exact same thing and same problem on my Old Isuzu Trooper 3.1 Too..... Check simple cheap stuff first......... Good luck i hope it's not Major