86 Corolla stalling when overheated.

Unfortunately, my mechanic is stumped with this one. Car runs fine in dry weather, idles kinda funny when it gets wet. I know what you're thinking, spark plug wires right? Changed them right away and threw in a new distributor cap and rotor for good measure, no change noticed.
The REAL issue is this, in wet conditions, when driving on the freeway for
a SPECIFIC time and/or distance, under constant pedal pressure, engine
steadily loses power and eventually stalls. If left off for a period of 3-5
minutes it starts up again but can only be driven off the freeway, on
city roads and speeds. My mechanic suggested a new fuel tank, since the
fuel filter appeared clogged, after the switch to a new tank, no change noticed, still the same problem!
Don't know if the following is of significance but I'll mention it anyway.
The air-cleaner compartment shows an almost constant pooling of water.
I've determined that it's coming from the AS (Air Suction) system, strange, since this is a valve that only draws air to the exhaust. In my
case, it spews an oily residue (clearly seen on the air filter) and water
into the air-filter compartment.
ANY suggestions will be greatly appreciated, thanks.

Humm...I donno, have you check the AFM and TB??