Exactly the same on mine! Will try swapping a new one as per @octo 's recommendation.My boost gauge has a similar issue . The needle slowly goes all the way up then all the way back down when it decides to stop working and never works again unless I turn the car off and on again (usually the car needs to cool down for it to work again) .
I tried changing the vacuum hose as well but it didn't help . If it was a vacuum leak issue I guess the needle would just seat on 0 instead of -7 anyway.
One interesting thing I notices is that you can recreate this fault by unplugging and re-plugging the gauge while the engine is running so I suppose the problem is just due to an intermittent bad connection .