This can be anything
If your board would not even try to read the flash, it cloud be a problem with oscillators not coming up, power leaking (e.g. if you have it connected to different system for example through serial port), wrong power sequencing etc.
What is unusual, that you can see CPU trying to read from FLASH.
Is the problem consist? I mean, if a board has this problem, can you easily repeat it or it is completely random? Is it always the same board (if the board has problem, it keeps having it, if it doenst have the problem, it never has it)?
Changing DRAM may influence more parameters, but it doesn't have to mean it is DRAM (e.g. it still can be power seqeuncing, reset timing, power consumption, soldering, ...)
Does the bootloader always come up (even on the faulty boards)?
Have a great day,