Can anyone tell me what is wrong with my 1998 clio S reg?

my temperature gauge isnt working properly, the needle just goes on red after 10mins of driving it, it hasnt over heated once though, i have replaced the thermostat and checked for blockages, there isnt any. i have realised that when the car has been driven and the needle is on red, the bottom raditor pipe isnt getting hot but the top one is, so the top half of the engine is getting very hot but the bottom part is freezing cold.
Does anyone have any ideas what it could be?