A.)Check the SD card on other phone.If the other phone also reboots then ,replace the SD card
B.) Sometimes Even factory firmware is faulty. Check samfirmware
,for the latest firmware.
Check your firmware in 'about phone' in settings or typing in *#*#4636#*#*. Most probably you have I5510DDJK4 or I5510XWKA2 or I5510JPKA1 firmware. Flash with whatever firmware you want and try again.