Sounds like you might have a bad plug/wire. An old trick was to take a small vacuum hose and stick it between the coil and the plug wire. Then you could ground out each cyl to see a misfire when putting a load on the engine. Hey it is better than some techs. They would just puncture the boot to test.
You could have one of several problems. I would take a spray bottle of water and spray the plug wires until wet. Next start the engine and have someone load the engine. This means put the car in drive with the e-brake on and feet firmly on the brakes. Slowly apply the gas while on the brakes to create a load on the engine. When applying a load to the engine, see if it starts missfiring. You might be able to see the spark actually jump from the wire when wet. Usually this means bad plug wires. Now if the miss is real bad, it could be a coil pack, which would usually mean 2 cyls will miss at the same time.
You can check the coil packs by removing the plug wires 2 at a time for each coil, and test the resistance while lightly tapping on the coil.
If you get a tuneup, I would suggest getting plug and wires at the same time. If the plug wires are numbered, meaning the cyl number is shown on the wire, they are original . Most likely they have been replaced at least once before. If you see the wires numbered, by all means replace them right away.
One other possible problem could be at around 40 mph, your converter locks up, and this could "chug" at those speeds with a slight load.