NO thermal paste at all? It's strange.
Anyway, you don't need new thermal paste if you don't actualy remove the heat sink.
The only normal concern is dust buildup and fan failure.
Check for dust under the fan. It's the worst place it can be.
Make sure that the heat sink is properly seated with a very thin layer of thermal paste between it and the CPU.
The thermal compound purpose is to fill any eventual air gap between the CPU and the heat sink. It's less conductive than direct metal-metal contact, but much beter than air.
The heat sink should be held in place over the CPU by a metallic clip or some other mean, like bolt like rods going through the mother board. It MUST rest flat and firmly on top of the CPU.
Electro
September 2011