96 monte carlo z34 cooling problems

I've been having problems with this for 6 months.The temps gage when engine's hot,at high speeds (100-120) goes up to 3/4 then rises to 'hot' when I get on I slow down on an offramp.I have to pull over and turn off the engine a few minutes than its normal again.I have no problems at normal city speeds.
I had the head gaskets (the works) done,new rad hoses,fan relays,thermostat & sensors....still overheats.
Someone posted the exact same issues.Replacing the waterpump he had with one purchased by the manifacturer solved his problem.Should I try this?
Another mecanic gave me a story about a cracked block....I'm becoming skeptical here.Need some advice please.
Thanks.