Initial screen with logo blinks sometimes with red screen appearing

Hi,

I'm trying to bring my wife's old iphone 5S back to life. The problem is that it's stuck with startup screen (gray background with logo) for a few seconds. Then it goes to black screen and restarts back to this initial screen. Sometimes at the end of the cycle the red screen appears for a second.

Following my search I replaced the battery. Didn't fix it. Then I found out that it could be some front screen assembly component that is damaged. So I replaced the whole front screen with all the components (as it wasn't very expensive to buy). Well, still NOT WORKING.

Is this a lost case as this point? I don't have anyway to diagnose the phone. And probably it does not make sense to spent any more money on it.

BUT if any one has any suggestion what else I could do - please let me know.

Thanks,

JD