Warning light comes on while driving in the higher gears?

i have a 05 renault traffic been having problems when driving at higher speeds in the the 5th and 6th gear. When driving between 70-80mph a light comes on which i belive is the glow plug light which comes on when you first start the van. The van then becomes sluggish and does not pull anymore untill you turn it of for a minute and back on then everything is back to normal untill it happens again. Can anyone help with this problem thanks.