Peugeot 406 power loss and speedo cutout

It intermittently loses power (with a slight jolt) and the speedo cuts out altogether. This fault can last 2 seconds or two or three minutes and then everything goes back to normal. It used to be caused by bumps or left hand corners i.e. roundabouts but now it’s happening anywhere. The problem is getting worse and Peugeot have connected up their computer diagnostic equipment to it three times but it shows no faults or error codes.