My mark 8 has a shudder that occurs 80% of the time but the other 20% it's smooth. It does it in any gear, any throttle condition and sometimes seems to shake at idle. Usually it doesn't do it at WOT but sometimes it shakes super bad at WOT. When it shakes bad it has a sort-of booming/rattle sound to it, could just be from shaking the whole car though. The transmission shifts great but is occasionally slow shifting into Drive, it's usually a quart low on fluid when this happen. I've check the converter bolts and they are great and the flywheel bolts seem to be great too. The only play I can find here is in the crank end-play. The rear main seal on the engine and front main on the trans don't leak. I thought maybe it was a miss-fire so when it was doing it at idle I disconnected head plug wire and each one produced the same change in idle leading me to believe that's not the problem, plus it has good power and no codes. The fluid is bright red but I think the previous owner used that cheap mercon/dextron fluid in it because it has a leak at the fill tube (appears to be an O-ring is bad, or at least I hope). I've been adding mercon V. I'm told they are compatible but I've always done complete flushes in the past. I don't want to waste my time/money flushing the system if the converter is screwed. Any advice is appreciated. Edit, it's a 1996 base model.