Adrian, it does sit in our garage all the time, but it is not heated. I would think at 55mph your drawing less gas, less rpm's and your in the D gear, compared to when your accelerating faster in 1, 2, 3, etc. at higher rpms's? I could be wrong. I can tell you when I step on it, the V6 has plenty of ummmf, and it appears to shift down and has no problem, just at cruise speeds. I think the problem is the tranny or rear end, not sure...
Sounds like it would be messy, heck sometimes when I change the oil I'm messy.
I'll probably run it to my mechanic friend, it treats me right..