Likely one or more of the spark plug wire is going bad.
Also it could be one or more of the spark plugs wearing out.
Or it could be the distributor/rotor is going bad.
Any one or a combination of all will give you a weak spark
across the spark plug gap.

When the engine is under heavier load, a problem/weakness in
your ignition system will show up because it is more difficult
to fire the spark plugs with a stronger fuel mixture in the cylinder.

If you have not replaced any of the above in the last 50k miles,
order plugs, wires, distributor rotor, & a distributor online
from a discounter and do a complete ignition tune up.