Hi, Symptom: The car shudders at low load under 55 mpg and if pressed at higher speeds may also shudder. No shuddering occurs in 1st or 2nd gear only 3rd and up. Likely Cause: Bad Spark Plugs or coils Bad TC -Open to other suggestions? Troubleshooting accomplished: I changed most of the coil and plugs except for 2 coils. I will change the final two before check spark plus and coils off the list. I also changed the fluid and man it was easy I just wish someone would have spent the 30-40 dollars and done it 50k ago. Neither of these repairs fixed the shuddering although the engine did run better after changing the lugs and coils. Planned additional troubleshooting: I will also attempt to remove the TC pin from the transmission wire harness to confirm a TC failure before going whole hog changing the transmission. If the TC is the culprit: So because I like to have an idea or plan for the worse case which is transmission replacement. Does anyone know which transmission 6F50 or 6F55 is the right transmission for a SHO? When I search ebay for a used transmission I find that they try to sell you basically any used Taurus transmission but that can't be right. Certainly the SHO has a slightly beefed up transmission. I have the 2012 Ford workshop manual for the SHO and I think it says in there which one is supposed to be the correct transmission but I wanted to ask on the FORUM anyway. I realize this is no easy job: The job is very detailed I know and will have to setup and scissor car lift to do it in a gentlemanly manner. Its worth it to me to change it myself using the workshop manual because I will know where I stand when it is done. I would also like to do a bunch of maintenance on other things that would probably not be done if I take it to a shop that just does transmissions (Paint, preservation, etc...). Also I dont really want to pay a fortune to fix this and its definitely not under warranty 150k.