Have you checked compression? It sounds like its not getting the right amount of something and compression would be the first thing I would check.
1991 Mustang LX 2.3L 5 Speed convert
1989 Ranger 4X4 2.5L 5 Speed solid Dana 30 front axle, 4.5" suspension lift, 3" body. ( blown up....)
Entire turbo kit waiting to be used again.