hi just joined i looked here before to try and get answer to your question it has been happening to my vivaro also . my coil light was lit as i lost the power at first it was intermittent but as time went on the light just stayed on and power was pretty awful . anyway its been to the garage and is all sorted now . the air box was loose but that wasnt the problem . 3 of the glow plugs were had it but i think the loss of power was caused by a relay which was something to do with the glow plugs i think . the mechanic didnt tell me the name of it but two mechanics i had seen said it would be a relay . i hope this helps mate im chuffed mine is sorted ..... mart