My main symptom is that the engine seems to be flooding while the car is driving. The quick fix that Ihave been using to get the car started again: remove the fuel pump fuse, start the car until it dies from lack of fuel, replace the fuse, and restart the car. I have been using this process for a few months now but the issue is becoming more frequent. Based on my research of the forum and internet, I'm think the best course of action is to replace the fuel pressure regulator...

The car has 225K miles on it and the full tuneup was completed recently. Since I know little about the fuel system and how it affects the engine idle, I feel like I should mention that for many years now the idle runs around 1500 rpm and coincidentally (or not? IDK) when that is the case the car does not shift to overdrive. Well, this all changed about 6 weeks ago...I was taking a rare two-hour drive and as I am going along the highway at 4000rpm I notice the tachometer start to shudder, then the car shifted to overdrive. In the moment I couldn't hardly believe it but I was very giddy and when I came to my next stop, the car was also idling normally (~900rpm). It turns out that this change is temporary--My car will only revert to normal idling and normal shifting to overdrive after driving for more than an hour. The engine has never stalled under these conditions. Maybe none of this matters, but I hope it does.

Thanks in advance for your support.

FPR replaced and car has not stalled since. Thanks again.

The fuel pressure regulator has a rubber diaphram that will crack with old age. Easy to check, the FPR has a vacuum hose that runs across the manifold and to the vacuum tree. Pull the hose off the vacuum tree and sniff it for gasoline, if you smell any or a little dribbles out, get a new FPR.

When my FPR died, it took a while to get the engine started and then at engine start, a puff of black smoke came out of the tail pipe. Much too rich.

As for a high idle, there is an air leak into the manifold. Check all vacuum elbows. Check the air hose from the filter to the throttle body for cracks. Low vacuum pressure will also cause the engine to run rich. A clogged PCV or EGR won't make the condition any better!
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Keeping it running is better than buying new

So, after the FPR replacement the car started stalling again and having no start issues. The fuel pump and the fuel pump relay were replaced, but intermittent stalling continued. The on-board diagnostic system is not responding.
We serviced the PCV system and replaced all vacuum lines.
Spark is still confirmed.
The problem remains that the engine is intermittently receiving too much fuel, sometimes at startup and sometimes while driving. Removing the fuel pump fuse and cranking until excess fuel burns off, replacing the fuse, then cranking again will start the car.

I'm not understanding why the engine is receiving too much fuel still. Does something need to be adjusted to send less fuel to the engine?

Thanks very much for this information! The weather is going to warm up tomorrow and Saturday, so I will keep you posted how things work out after that!

Besides vacuum leaks, your idea to replace the fuel pressure regulator makes sense. Keep an eye on the seals and apply some Vaseline to help them seal.

I'd look for a vacuum leak first. That old rubber has only so many years
of service life.

Bill