Corrosion around U11 could have spread and not be visible. Check pin 40 on the CPU. Is should go from low to high after the 5V has stabilized. This is a normal boot. If Pin 40 stays low, then the LED will stay on. It sounds like it is booting and then rebooting.

U11 PIN 34 is also the RESET signal. Check your work around this pin.

Sounds to me like the RESET section is not holding 5V. Check Q1 and Q5 - these are easy fixes and cheap to replace.

Before looking at the board - Could you eliminate something - Connect up only J4 to the MPU and see if it still reboots without the other connectors.