Misfiring at idle and under load?

Hello,

It does sounds like the issue is weak spark plugs. As you mentioned they are the original plugs with over 85K miles.
Do an inspection of the plugs, check the gap. It should be at .060 in.
If you have excess carbon buildup it is time to change them.

https://www.2carpros.com/articles/how-to-change-spark-plugs

Is your check engine light illuminated?
Typically with misfires the engine light gets triggered.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Also if it has not had a tune up, it will be a good idea to perform an engine tune up. Typically on a tune up you will have to change the plugs, wires, air filter, fuel filter as well as cleaning out air induction components like mas air flow sensors etc.
Here is a guide of a tune up:

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Please let me know of any questions.
Thank you.