transmission/sensor problem?
 

okay guys heres the deal im going to describe this the best i can. so a few weeks ago when i was on the highway driving at about 60-70 mph for a while and i started to notice that if i let off the throttle that my speedo would drop down about 20-30 mpg then go back up to where it was at. at this point it wasnt that big of a deal and i just thought that maybe my cluster was going out because it wasnt making the trans down shift. so i never fixed the cluster because my rpms would stay the same. a few days ago it was acting funny when the speedo would drop so i thought maybe it was a sensor. a few hours later it became alot worse. so i went to autozone and picked up a vehicle speed sensor (vvs) the one that goes by the right axel. i did that last night, drove around seemed to do the trick. as i was on my way to school this morning it started to act up again so i turned around and came back home, and went to autozone again and had them read my check engine codes. it pulled the right read speed sensor in the hub which i knew about before, and 02 codes because i dont have a cat. and he also said something about critical malfunction. anyone know where i should start? im out of ideas, and any help would be great.