There is no known issue with memory leaks, though it is a possibility.
This sounds like it could be a heat problem. If a fan is starting to die, your CPU or video card could start overheating when stressed, and start slowing down, which could gradually get worse as the fan dies. Exiting the game or rebooting would drop the load and allow things to cool off (had this happen to me with a CPU fan years ago; at first the lockups seemed like a software problem, as it generally happened with lots of browser windows open, then gradually got worse).
Try taking the cover off, if you have a desktop computer, and see if the CPC/GPU fans seem to be going slow/intermittent or making any grinding noises (assuming they are not covered). Point a fan at the open computer (or at a laptop elevated a bit on a hard surface) and see if you get the same behaviour with the game.
There are also programs that can read and monitor temperature and fan speed info reported by the motherboard (I think that has become fairly common with remotely recent motherboards).
