arduino UNO forgets the code

I don't know why but today my arduino UNO started to do a weird thing. I upload blink example, all goes ok, then I unpulgged it and return to plug in and seems that arduino is restarting forever, L led blinks around 3 or 4 times very fast and do a pause. Tested in W7 and ubuntu 10.04 with same results, arduino uno R3.

I had something like this once. My problem was solved by programming the bootloader into the Uno. I used a USBasp programmer for that.