1990 Mitsubishi Galant 2.0

I have a 1990 Mitsubishi Galant that
has almost 300,000 miles on it.
It's been one of the most reliable car
I've ever owned in my 50 years of
driving.
I started it up a week ago without any
issues. I got about 6 miles before it
started losing power.
I pulled over to the side of the road as
soon as possible and noticed that it
was surging at idle, and it then
sputtered and died.
I opened the hood and inspected the
battery cables, coolant and looked for
loose connections and fuel leaks.
I then got in the car and tried to start
the car.
It fired right up and was running well
for about 3 minutes before it started
surging again.
The car could not get up enough RPM's
to move. Pressing the accelerator just
made it surge harder and spit and
sputter until it died again.
I had it towed home and started to
work on it.
I disconnected the battery and let it sit
overnight. I went out the next morning,
connected the battery and it started
right up and idled nicely.
I drove it up the road about a mile (up
hill) before it started surging again.
I rolled back to my house and tried
again the next morning.
Same thing happened.
As far as I know there is no way of
doing a diagnosis on a 1990 Mitsubishi
Galant 2.0, so I replaced some parts
(that after 300K trouble free miles) I
thought should be replaced anyway.
I replaced the TPS, fuel pump and filter,
cap, rotor plugs and wires, and looked
for obvious signs of broken, corroded
or loose wires.
Nothing has changed.
It starts, idles nicely and accelerates
like new for about 5-8 minutes, then
looses power, surges and eventually
sputters and dies.
Please help!