Can you provide the definition of Node. What exactly is List::sortlist trying to do? as you do a data comparision first then enter a while loop which does more comparisons? You say that the payload (data) may be huge - yet you are doing comparisons on the data - so this data must be of a type for which the > and < operators have been defined.

Is this a homework assignment? If it's not then why bother with writing linked lists? Why not use one of the STL containers? If you do need to write your own list code, why not create the list sorted in the first place? As Paul mentioned in post #5, why store a huge payload of data in the linked list instead of just storing a pointer in the node to a dynamically created object?

Have you tried drawing a list of a few nodes on paper and then using the paper list walk through your code moving pointers etc on the paper exactly as the code says? I found in the past that helped me with pointer issues.