2005 Vauxhall Vivaro lacking power

starts fine idles fine but will not accelerate fast until it reaches 2500 rpm then screems along untill i change gear ,after 4-5 mins the glow plug light comes on and whatever speed it is doing it will not accelerate higher and starts to slow down the light goes out after 2-3 mins then it is very sluggish again once over 2500 rpm it roars away but before that its extremely unresponsive
driving me nutts ?