I have had this van for years and replaced the transmission on it about 7 or 8 years ago. It's really odd, at first I thought gas pump was dying on it again, but realize its seems more of a shifting issue. It only does and not every time when the van is first started and your taking off and its shifting from 2nd to 3rd gear. It will all sudden almost like its stalling but jerks and speedometer goes showing around 80MPH then 20 then back to 45 50 MPH once shows miles right it shifts and continues on stop and go to shift perfect :O. Any idea what could be causing this? I was originally thinking tranny going, but fact it shifts fine and way speedometer reacts I was wondering if this is a sensor of some sort?

Thanks in advance.