computer crashing

i initially have a ddr266 kingmax 128MB, an msi motherboard with a VIA chipsets, and intel P4 2.26GHZ. i want to upgrade the memory so i placed a 256MB ddr266 but of different brand name. it works fine but the problem is that after 1 hour or more than 1 hour, the computer will restart, and then will restart again, and again… what must be the problem????