I haven't had any problems with any of the Thermarest mattresses I've owned. Unless your talking about the effects of temperature fluctuations in the air affecting the 'fullness' of the pad?
I have 4 Exped and 1 Klymit and have never had an issue with any of them going flat, other than a little pressure due to temperature change, as was previously mentioned.
Sea to Summit Comfort Plus Insulated Pad.
Has a well thought out and comfortable mattress coil design w/dual layers (in the event you deflate one layer you can rely upon the other.
Also has a unique recessed multi-function valve system.
So, I did a little test the other night and blew up a thermarest xtherm and a klymit static v and put them in the basement which is cool but not bad. The klymit held air better but both of them lost air. I always have to blow up both of these when I come back to camp in the evening. They don't go totally flat but it just seems to me they should hold air better. Maybe I'm expecting too much.
But for god sakes man, for the money they charge for the things now a days, one would think they would hold air better.