I have a 1998 Subaru Forester L in which I just replaced the powerplant. Car ran great for about a month than developed a hesitation when driving at all speeds. Replaced spark plugs and had mechanic reset idle controls, adjust throttle cable and clean out maf. Car was also stalling at idle which does not happen now, but still hesitates while driving. I'm getting no cel's and mechanic gets no codes to read. Any ideas?