I have a 1999 Subaru Outback, and am having intermittant trouble with

[grantepritchett]

accelerator response? On occasions when accelerating from either a standing start or from a slow speed the revs fail to increase. After aprox 5 seconds the revs come up. On 1 occasion a continuous dull popping sound was heard coming from the inlet manifold.

 

[kelly_f_1999]

dirty fuel system a dirty air filter or bad spark plugs