2001 Subaru Outback Hesitation

I have a 2001 Subaru Outback wagon. Manual Trans. With 104,500 miles. Since about 95,000 miles (only when the engine is up to operating temp.) It will hesitate and "buck" if given gas too quickly. It is as if the gas fuel mixture is not correct and the engine is fighting to adjust. If I accelerate solely then I don't notice the bucking.

Car idles fine and accelerates well when the engine is below normal operating temp. No check engine light has come on. No codes are obtained when the vehicle is checked.

Fuel filter has been replaced, plugs only have about 20,000 miles on them, new timing belt, water pump, temp. Gauge etc. Were put on this weekend. No improvement in car performance.

Any suggestions would be helpful.