My 90 350SDL always seemed slow on acceleration. Rebuilt injectors, new timing chain, set timiming, replaced all filters, checked mass flow pot, water temp sensor, tps, turbo press; all fine. ALDA all the way out. Still poor accel. Went like this for months, then for two weeks ran like a bat out of hell for no reason, geat accel and large cloud of smoke from rich fuel when I jumped on it, still good mpg. Now its back to the way it was. Any ideas welcome

Only thing I can think of is intermittent leak to the ALDA from the manifold or an intermittent leak internal to the ALDA.

Remove the ALDA and see if the performance returns. Then you can chase it backwards to find the culprit.

The ALDA should hold pressure if you've got a Mityvac that will do pressure in addition to vacuum. DO NOT apply vacuum to it.