over heating

[spear_]

have a nova 1.5td runs fine on my drive for 40 mins @ normal temp once i get on the road and start to do some speed say 70mph after a little while 10-15mins the temp from cold is getting towards the overheat mark on the dash any ideas there are bubble escaping from the expansion tank but have run the car for 40mins and bubble are still there tried flushing the system still doing the same was thinking of getting head skimmed and new gasket but thought i would try here first
plz help this is doing my head in
cheers in advance