Jack...have you seen/read Capt Jack's thread -
Automatic gearboxes at 50mph - in General Volvo Motoring Discussions? Makes for interesting reading as your car may be perfectly fine.
I can't comment further on the AW50 (4 speed) as we only bought ours last July and it's at our place in Poland so don't get to use it as much as we'd like. Although, it drove faultlessly for 2k kilometers across Europe, I'll keep a note of it's behavior this summer.