Ok, recently, my wife and I have started to notice that we have a slight delay in our throttle response when the light turns green at any given intersection. takes about 1.5sec before it reacts to the gas pedal being pressed. the van was a little low on oil, so i topped it off. wasnt bone dry, but a little low. trans fluid is fine, no noted discoloration or darkening. trans fluid has no burnt odor to it. generally try to run injector cleaner through the system about once a month. just about every fuel pump in the area "may contain up to 10% ethanol," so i try to keep the cleaning going. any suggestions to help me out? could it be the spark plugs? i know their time is coming soon (about 250mi to go), but is that a possibility? since the last tune up, most of the miles acquired were highway. now, we lag. im at a loss here. check engine light is intermittent giving O2 sensor code, but when i flush the injectors, it goes away and returns 2wks later. maybe O2 sensor is about dead and causing this? any help would really help!! thanks!

vehicle info: 96 windstar gl, 3.8L auto