Watch the levers when you lock the door...

My old Sierra LX's did this, and it's usually the passenger front that appears to sieze. Operating the lever (opening the door from the inside ) is usually enought to sort it.

Also, don't rule out the boot lid's lock - if it's slightly 'out' it won't lock and will re-cycle the system. Doors that aren't shut properly or misaligned can also induce this problem.