Skipping, Missing, hesitating

I drive a 1999 Ford X V8, 189,000 miles and it’s been a great car. Lately tho, and in particular when the temperature is above 90, it begins to miss, hesitate, then jerk forward all at once. Some times, when the accelerator is depressed, NOTHING happens. No response form engine at all. This can last three seconds of so, then the car will lurch forward. I have done a 180,000 mile complete tune up, a, fuel induction system service, replaced the Mass air flow sensor and
checked all points, plugs, new air filter, etc. Still, the problem persists. Any idea what I should tell my mechanic next? thanks for any advice. Much appreciated.

Does it get worse on a full tank or does that not make any difference?

That kinda sounds like a vacuum leak. Have you noticed a rough idle or a higher than normal idle speed? Is your check engine (MIL) light on?

What’s a “Ford X”?