Well about 2 weeks ago the temp gauge would randomly climb to just above half the slowly fall back down, so i had a quick look under the bonnet while the car is running (and up to temp) and i had noticed a hissing from the drivers side end tank on the radiator, so i put some seal up through it, new coolant, thermostat and bled the system and it seemed to fix the problem for about 3 weeks, and the other morning the temp gauge had just gone over half so i pulled up, popped the bonnet and just felt the radiator hoses to see if the actually were hot, the bottom radiator hose was warm (i could hold onto it) and the top hose was coldish and the radiator cap was still cold...

Does anybody know whats going on with this?

Any help or suggestions appreciated.
Cheers
Southy