Fluctuating MP on rear engine

[Richard]

I had the same problem with my rear engine. Turned out to be the turbo seperating from the center section and needed to be replaced. I agree that it could be the wastegate, but the range 4" = 2 psi would suggest the turbo. If the turbo is fine (end play, etc) then I'd look at the controller on the top of the engine. Those old turbos are also suspect to coking on the bearings. The turbo should spin freely and without effort. If it seems difficult to spin or a rough feel, then you need to overhaul the turbo.