I may not be sure but I had a 86 sprint 330k+ miles,(I love that car)
and if this condition isn't new to your car, it could be related to a malfuntion in your turbo. Not turning smoothly and creating too much back preassure when in high revs. Its only a poor 3 clyr! But if your engine has been rebuilt what about the turbo, needing more attention than the engine itself?
Then again this does not really apply to my field. I use turbos on desiels.
(I never knew sprint was made turbo.)