Hi GarryE, there are two components that make an engine go - fuel and electricity. How well a car goes does depend on the degree of efficiency of the various components. So a car might go well, or just go.
Your mechanic has had the car on a tune-up machine, and everything checks out okay. Therefore, one would assume that plugs, high tension leads and most electrical components are in good shape. Keep in mind you have the coil, distributor, leads, suppressors, distributor and cap and the ignition control unit.
That then takes you to the fuel system - Fuel Filter - how long since it's been replaced ? Fuel Pump - has your mechanic checked the amount of fuel flow and is the pump breaking down partially in the higher rev. range ? How old is the pump, and does it hum loudly ? A good pump will be almost silent in operation. You can also disconnect the electric plug (with blue plastic cap) that connects to the switch that is located to the right of the fuel distributor on the Inlet Manifold. Disconnecting that switch ensures a continuous flow of fuel even with engine not running.
There are a few things that you can work on - all of the above assumes that you have the K-Jet Fuel system. Keep in touch, and try and supply an answer to some of the queries I have raised. Regards Styria