2005 S60 Tranny Issue? Or am I nuts?

[sporald]

Heya. Hoping someone out there can help keep me out of the service station.

I'm getting an engine code P0811 and Service Tranny message on a 2005 S60, but only when I hit about 60 MPH. Car's running fine, shifts fine, in manual or auto mode, through all 5 speeds. Even when I'm in 5th gear, it's running fine, until about 60MPH, then check engine, service tranny, etc. It'll run fine while that's happening, but if speed gets reduced to about 45-50MPH, it'll pop into limp mode. Clear the codes, and it's running fine again.

I recently replaced the input signal sensor on the tranny when it went bad (with the help of this forum), and that's the last issue I had with the transmission. Fluid looks fine and full.

I'm really have no other indication that something's wrong with the transmission until I get on the freeway and get it up in 5th gear around 60MPH.

Any idea what the issue could be here?

Thanks again in advance.