I am having the same issue (except I don't drive quite as fast) but my tach has been dancing around a bit. I think from researching this site, that it may be your alternator. Other folks have had their alternators act up when they've been driving their cars a lot which AC or big stereo systems.

When my car cut out...actually it cut out on my daughter who was learning how to drive and really freaked out, it was after a 40 minute car ride with AC and music blasting...so I just cut the AC off, cursed a bit, and it fired right back up and made the trip back home.

It sounds as if the regulator in other peoples alternators may be going bad. I got a used alternator but haven't had time to change it out yet. But my alternator was already acting up (lights were flickering) so that is my issue...but it seems to be a similar problem as what other people have written about.

 

i had that problem taking an off-ramp ... NOT FUN. it was the alternator(in my altima)

 

undo the negative battery cable and trace it down wherever it goes down into the engine bay. wherever it is bolted to the chassis is a "ground point". undo the bolts and sand the terminals with ~400 grit paper until they are clean and shiny. also sand the point on the chassis where they touch. if those points are painted, sand off the paint before you reattach it so that you have a metal-to-metal contact. smear dielectric grease (from autozone etc) on everything. also put it on your battery terminals after you sand those.

here are pixx from when i did mine on my starion: HAMBURGER

also do the same thing with the fat "charge wire" that goes from your alternator to the positive battery terminal.

if that doesn't solve it, your car will be happier anyway cause people neglect that stuff usually & it's important.

 

undo the negative battery cable and trace it down wherever it goes down into the engine bay. wherever it is bolted to the chassis is a "ground point". undo the bolts and sand the terminals with ~400 grit paper until they are clean and shiny. also sand the point on the chassis where they touch. if those points are painted, sand off the paint before you reattach it so that you have a metal-to-metal contact. smear dielectric grease (from autozone etc) on everything. also put it on your battery terminals after you sand those.

here are pixx from when i did mine on my starion: HAMBURGER

also do the same thing with the fat "charge wire" that goes from your alternator to the positive battery terminal.

if that doesn't solve it, your car will be happier anyway cause people neglect that stuff usually & it's important.

While you have the terminals off, loosen the tension on the belts and check all the pulleys by spinning them. If they all spin without any weird friction you're in good shape. The only reason I would say try this is because you said you were having problems with the power stearing. And when rpms get low you said you notice the car acting crazier (sometimes this means an accessory off the belt is going bad). Anything that spins weird should be checked out a little bit more.