water cooling system ????

i have a vectra b facelift 2.0 di my heating goes cold when the car is just running when i rev the car the heating goes red hot the water in the water bottle keeps building up loads of presure and the tempreture gage go up an down like a rocket i change the thomostat but i have no idea a few ppl have said to me could be a big air lock how do i solve it

steve
February 2009