The 89 will miss, chug, backfire (all of the above) when accelerating or going up any type of grade. Straight away or down hill 85 - 90 no problem. Plugs and wires replaced, 15K miles.

Check engine light?

no check engine light

sounds like problem i had with MAF, but I had an intermittent SES light

Check your PCV valve, IAT sensor, and MAF for carbon.

Other than plugs, what regular maintenance items have you had done in the past, say, two years?

Off the wall question...what brands of plugs/wires did you drop on the car?

For the fuel is it multi port or throttle body?

Plugs and wires - delco
fuel filter, pcv, and air filter - fram
15K miles
fuel pressure - 45 psi (never changes even when accelerated by hand)
no carbon build ups
check all sensor connections for corrosion
no check engine light
same problem with the engine hot or cold