I keep falling through the ground in Hellfire Peninsula.

And it wouldn't be a problem if it happened rarely but it happens so often that the game is unplayable.And I don't get stuck when I fall but I die.

Clear your cache and it won't happen again

I've not run into this problem in HFP, but in Feralas I have. When walking uphill (riding a kodo if that matters) I seemed to have found many places in the terrain that catch, hang, and clip. I've fallen through the map in Feralas many times.