Rst Pin going low 2 times per second

I all. I am working on a bruhsless motor applicaiton with a STM32F103RCT6 MCU. I got twice the same problem on my board which ended first time by changing the MCU. Before removing a second MCU, I would like to understand the problem. My board has a self holding feature on the 3.3V. Hence to flash the board I use to had a jumper to hold the 3.3V. I forgot once to put the jumper ON and try a flash. After that, some pages were write protected. I manage to remove the protection, perform a mass erase and reflash but my MCU is now resetting 2 times per second with the RSt signal going low. There is no issue on 3.3V. Does somebody has already had that kind of issue ?