Plug wires breaking down under load.
Fuel filter-flows enough at low load conditions, not enough for power.
EGR valve sticking.
Sparking Plugs with bad or wide gap breaking down under load.
TPS sensor bad fooling the ECM into thinking you closed throttle.
MAP sensor out to lunch intermittently, leaning or enrichening mix.
Pull the codes, and see what you find before spending $$. The TPS can cause the trans to drop out of OD if yours has an AXOD.
Again, pull the codes first.
ps. you could get the 2.5 out of a Taurus, and have a mild sleeper...
that'd be fun, lots of torque, too.