Thought I would add to this thread. 2000 Outback 73,000 miles all stock. Developed a stumble under load but light throttle plus backfire into the intake. Wouldn't set a code but got the light to flash a couple times. Car had new T belt, plugs, head gaskets at 58,000 mi due to collision damage.
Anyway, did some research on the I-net and was getting frustrated with all the possibilities. Decided to try a new set of plug wires. When I pulled # 3 from the coil pack the terminal was green. Cleaned it with scotchbrite, installed new wires and it runs perfect. BTW, and strangely, after this the engine light was on. Said misfire cyl 2 even though the bad wire was #3. Cleared it and it hasn't come back.
Hope this may help someone. Sometimes it's so simple and right under your nose.