Over Heating?

I have a toyota celica 1991 2.0gti, just recently it's started over heating rapidly and kicking out excessive white smoke, on tick over it operates at normal temp, but 5-10 minutes into driving it, it rapidly overheats, if i slow down to 10-15mph it cools back to normal temp, soon as i go 30 or over it over heats and the temp dial jumps up and down, i'm getting major steam coming from the radiator, yet both fans are operating fine, i've replaced the rad cap and thermostat and flushed the rad out and the problem is still happening, the only things i can think of is the water pump or rad.
any help would be useful thanks.