Stalling, cant get over 55MPH!!! Help (Long post.)

This is my buds truck. Awhile back he had a burnt VC gasket and wiring harness. Replaced VCG, new GPR, and glow plugs on the passenger side, and new pins on wiring harness(soldered the wires). Income tax comes in and add Tranny savers, downpipe, cat-delete pipe, and open air intake. Truck ran like a champ for about 2 weeks, then hesitation, stumble and trouble starting. I go over to check it out and no problems, ate lunch drank a 12-pack drove around the ranch and no problems at all. This week he drops if off with me and truck dies about 100 ft from my house and will not start. Swapped IDM and mine wont start with his, and his will start with mine. Problem fixed. I drive it around and no problems at all other than it seems the RPM's build up a little slow than what I am used to. He comes to pick it up and bang it dies again and will not start. Swap CPS and truck runs and starts fine everytime. He takes off and calls me back anything over 55-60MPH and truck seems to defuel and white smoke out of the exhaust. At 55 MPH/2100-2200 RPM truck runs fine over this RPM it stumbles, does this in 4rth gear also. Truck is a 5-Speed and a 1994 and not OBD compliant...Scanner will not work on it. Any ideas?? So far new injectors, Glowplugs, gpr, idm, CPS, and added DP, cat delete, and open air intake. His oil was very dirty and I told him to pull the #9 fuse and change the oil and call me back today.

Thanks,
Art