Hay fox

I have an ECM Giotto from the oldest model with the half-moon gauge, and i add to the machine a pressure gauge for checking the brew head pressure and I noticed on a strange problem-
When the machine is hot and I brew coffee the pressure stand on 8.5 and start jumping(not stable) it's jump from 8.5 to 7 and jump back and all the time it's jumping...
I was try to check my opv by open him and clean him up, but with no luck with that.
When I check with a blank filter it's stays 8.5 bar and not move.

What can be the problem? Does it is the gauge that may be damage/ or the opv?

In the first's attempt it was great with no problem.
You can see it in the the movie:


I change the boiler gauge too, for the same gauge.
I was adding the brew head gauge by replacing the angle that connect the hx to the head in a tee(like in most common machines)

Thank you all

Here is a movie and you can see the problem


Look in the gauge how is going crazy

Plz help me understand why it's happen

See Pressure pump gauge going crazy for explanation and possible solution:

jasonmolinari wrote:My Isomac Tea exhibited a similar pressure gauge flutter.
I took the gauge and connected dampening tube off and applied heat to the tubing carefully with a blow torch. Water sputtered out the end of it. Did that until it seemed dry.
that fixed the problem...

Careful not to get too close to the gauge with the torch!

For reference, see loosely related thread Eliminating needle flutter of DIY portafilter pressure gauges.