Misfire under acceleration, then stalls at idle

Ok, I really need some help here.  This is the problem...Starts fine, runs smooth.  During heavy acceleration, usually on the freeway during passing, the engine suddenly feels like it, "she can't take any more!", usally around 4k RPM.  This is followed by the check engine light flashing, if I continue to try to accellerate, the light stays on.  Shortly after the incident, if I exit and stop the car the engine dies, it won't keep an idle.  I have to put it in neutral and rev it for a while, then after some time, it will idle again.  Using a code reader, I've obtained a Cyl 4 misfire.  This is what I've done so far, in order to resolve this issue:

Replaced plugs and wires-No effect
Swaped Coils to see if Cyl misfire would follow, stayed at Cyl #4, ruled out Coil
Used a toothbrush and carb cleaner to clean intake butterfly-No effect
Changed cats and O2 sensors-Dramatic change, power was great and MPG impoved, but sadly, about a month later, the problems all came back.
Replaced timing belt and tensioners-No effect
Replaced Fuel Filter-No effect

As you can see, I've done quite a lot of diagnostics to fix this, the dealer said, "there's water in your tank" but I think they are on crack.  The only time we noticed a changed is whan the cats and sensors were changed.  But why would the problems come back?  Any help would be very greatful, Thanks guys.

Yeah really man I was going to say 02 sensor.  Hummm how the spark plugs look when pulled?  wet? lean?
 
Water in the tank hummmm--possible water can kill your 02 sensor, that would be alot of water though.
 
When did this problem actually start?

I had the identical thing happen with my 97'
 
When I had the timing belt changed the last time,the mechanic didn't tighten the harmonic balance bolt and it slightly wobbled out the key way..At idle the crank sensor would pick up the slight fluctuation as a misfire and shut down the engine.Might check it.