Why would a HONDA ACCORD V6 1998 RPM go down after driving 15+minutes?

So while driving, this has happened a few times now (this is when I'm driving to work, which is about 25-30 minute away). I will be able to drive to places like the store to get food, and I'll be on the road about 10minutes or so, without much problem; however when I go anywhere that's roughly 15 minutes or more away what happens is I'll be pressing on the gas pedal (going about 40mph) and I'll notice that my car will slow down even if I'm pressing the gas and the RPM will go to 1 and almost to zero (normally its at 2-ish when going at that speed) and while pressing the gas it will usually be 10 seconds until it climbs back up again. This is always near the end of my drive and I'm almost at my destination.

The first time this situation happened, I was at the light and was speeding up again when it went to green, got to about 20mph and it kind of stalled and wouldn't pick up, then shoots to 40mph. Then about 5-8minutes later it the RPM goes up & down sporadically, sputters for about 10 seconds and then just goes back to normal and I reach my destination 8-ish minutes later (including lights).

I've been hearing everything from bad spark plugs, to being my transmission, fuel pump, fuel filter, air filter, needing a new fuel injector, to needing just a fuel injector cleaning, blah blah…

The mechanic looked at the hood of my car and said that everything looked good, checked the oil/fluids and those were all good as well. The check engine light only showed "emissions" problem…

Added (1). When he and my mother drove my car they didn't feel anything out of the ordinary, but i figure they aren't driving far enough; cause it doesn't really happen until i've been driving for awhile. I already had to spend a ton on new tires and a new axle/cvjoints… And i can't waste anymore money just having them say "oh MAYBE it's this"… T_T
I haven't changed the plugs since I've got the car; i'm at 160k miles, I got it when it was at 138k, 4 years ago.

Added (2). though when they looked at the plugs, i was told they didn't even look warn…
I'm a grandma type of driver so I don't drive recklessly or anything…

Sounds like the classic Honda EGR blockage. The intake manifolds require cleaning off the car.Honda suggests cleaning them on the car but particles will remain and get sucked into the motor. I did this repair myself on a Civic intake manifold. If you do not fix the problem the head gasket will fail due to overheating.Spark plugs are not important I think people just relate to spark plugs.People think they are matches for cars and people like to play with matches. This job is old fashioned car repair that is rebuilding not swapping parts.Heart surgery is like rebuilding but doctors get medicare payments.In Japan there's a law that cars are scrapped when carbon builds up so they are not concerned.

I would guess that some electrical component is failing and causing the problem when it heats up. The emission failure codes might give you an indication of why the engine is not running properly.

Check you muffler, exhaust pipes and catalytic converter for carbon build up/blockage. The usual build up and blockage occurs at the catalytic converter and muffler. If your catalytic converter and muffler are bolt on type it is easy to trouble shoot. Remove the muffler first and drive the car for long periods. If it improved then replace the muffler. If not reinstall the muffler and remove the catalytic converter. Do a test drive if it improved then replace the cat converter.