We do have ImageMagick installed on our server, and the option for this is selected under Global Options.
Many, many users have experienced the same problems. As I say, I've spent an age trying to get to the bottom of it, but never succeeded!
Although our maximum width and depth were set to a maximum of 5000px each, I have just reduced them both to a maximum of 1500px each.
Furthermore, our PHP memory was set at 32mb, which was the server-level default, but I have now arranged for this to be increased to 128mb - slightly more than suggested.
Can you think of anything implement before I get some 'guinea pigs' to test it?
Finally, is it still worth putting
into pp-inc.php even though we're not using GD2?