I have a 1990 mazda protoge with 144000 miles.
This has been ongoing but is getting worse.
When started cold It runs great but when it gets
to normal operating tempurature it starts
missing @ low rpms and also when it's put under
a load (going up hills or accellerating).
On flat ground at a steady speed it runs smooth.
I've replace the plugs and wires and that helped for about a day.
It acts like the older carbed cars with a stuck choke.