I have a 54 plate vivaro. Recently when I’ve hit 70mph or over it’s started ,the stop light and engine stop light flickers, and the temp gauge bobs . If you slow down this slows down or stops. I’ve had cam sensor and another sensor replaced but this hasn’t solved the problem. No fault codes come up when plugged in. Has anyone any idea as to what could be causing this? The van seems fine and running well.