Strange, I got one of those last month and it had the exact oposite fault..

Initially it worked fine but after a few minutes of use, once it had warmed up.. the auto-shutoff stoped working and it just pumped away non stop.

In the end I actually helped the suplier fault-find it (I repair moving lights for concerts for a living) and fixed that one for them.. they then gave me a new one and returned half the price to me for the "work".. wasnt a bad deal!

The preasure sensor is a really cheap chinease relay and when I went online to look at the manufacturers website and specs it was pretty clear that they wernt the best....

well if you've taken it back to LHS no more need to worry..

but yeah I'd agree.. looks like the preasure is being measured properly by the relay and it is shutting off.... seems more like a bad gague, maybe dirt on it.. or badly drilled hole somewhere or leaking where it should or not alowing air to pass where it should.