My 30SI had a similar problem only it didn't revert back to normal pressure when switched off. What was happening was the stopcock on the fill pipe (seperately located in the hot press) was faulty and was allowing too much water into the system. I replaced this valve and there is a drain valve under the boiler, to which you can attach a hose/tube and open the knurled knob letting water out until the pressure drops back. Although 1 Bar is the ideal setting, water when heated will cause the pressure to rise slightly and 1.2 or 1.3 Bar is not really a problem. I am open to correction by any Engineers out there. What you are seeing with the pressure falling back to 1.3 seems a little different. Hope this helps in some way.
Ger Walsh
March 2007