It's beginning to sound like you've got internal engine problems. Next on the agenda needs to be a compression test, or a cylinder leakdown check. There's also a very good test that Sun used to recommend called a "power balance test". The engine is run at a fast idle on only two cylinders, with a steady throttle set by the curb idle screw. The RPM and manifold vacuum is recorded when running on each pair of companion cylinders. Low powered cylinders are much easier to spot using that procedure, compared to a compression test.
Also, have you done a valve adjustment, and checked for flat camshaft lobes?
Jerry
