xr7cougar1968

As of lately my 89 2.3 mustang has begun running very strange. The first thing I noticed tow weeks ago wasat highway speeds 40-60 mph, while maintaining a constant speed the car would suddenly lose power, but would return to normal when I tried to speed up. The next thing was when slowing down and coming up to stop signs the engine would start shaking like it was going to quit... again, until I gave it the gas which would bring it back to normal. And now the last few days when I leave the car sit and idle it will leave a black spot on the ground behind the tailpipe that looks like unburned gas. The check engine light doesn't come on either. Does anyone have any ideas of what might be going on? My first thoughts where O2 sensor and/or thottle position sensor, I even thought maybe something whith the egr valve. Any help would be appreciated, Thanks.