Blinking glow plug warning light

Hi !
I hope someone can help.
Alfa Romeo 156 1.9JTD, 1999, 236 000 km

Some times imeddiately after the engine starts the glow plug warning light is blinking for about 20 - 30 seconds.
In the manual says that is a failure with the diesel preheating system but nothing else.
The near-by garage checked the glow plugs and they are fine.
They recommended going to the dealer.
Any ideas what could be ?

Thank you !
Mihai