I'm in a lord's party and he's just gone through 5 loops, travels on land to old oak harbor and then takes a ship right back up to lannisport and repeat. Asked him what in seven hells he is doing and Ser Gregor said he was going to visit some maiden in faircastle.
Edit: also it seems that the mountain prefers to use a dagger when he isn't mounted instead of the great sword on his back.
edit2: I removed the dagger from the mountain's inventory so now he'll use his great sword. unfortunately he can't use it with one hand though. You should give The Mountain his own unique greatsword that he can use one handed. I think ACOK did that, one of the few things they got right in that mod! ;)