What tubing and what fluid temperatures?
Also, when you say that they're coming undone, is the tube coming off, is the compression ring falling off, or is it just leaking?
If it's just leaking, then make sure they're tightened down properly.
If the compression ring is falling off, make sure it's tightened down properly.
If the tubing is coming off, make sure the tubing is properly seated, and make sure the compression ring is properly tightened.