loss of power and rev counter stops working,seems fine when cold?

i have a 2001 vauxhall vectra ls dti.
the car lost power going down motorway and the rev counter stopped working managed to get to serviles i left car parked whilst considering best way to deal with it IE RAC.how ever i thought i would give it another try.the rev counter was working again and the car drove perfectly.so i decided to chance driving home problem started again after about 5 miles. managed to get car home safely as roads pretty quite .oil and water have stayed fine through out any ideas please. ?