A friend had a similar car with a similar problem. After doing all you have done and more, we concluded the torque converter is the problem.
Other years and models with basically the same engine have a torque converter that lets the engine rev to about 2400 rpm. At 1500 rpm not much is happening in the engine.
Don't mess with the pump unless the chain is very stretched; and if that is the case, replace the chain before attepting to adjust the pump.