iPhone reboot when I lock screen

I suspect that the phone you purchased was already damaged, probably related to whatever caused the the original screen to break. Depending on how you bought it (eBay?), you may have some recourse if it hasn't been too long.

Without knowing the history of the device, it could really be anything. You will need to isolate some variables if you can. Are you certain the screen you installed is a good one? If you bought a low-cost screen, it could very well be the problem. If you have access to a known-good screen, I would try that first.

After that I would disconnect the front & rear camera and sensor flex. That leaves only the battery and display connected to the logic board. Turn it on and test it over several days. If your problem goes away, then re-connect one item at a time until you find what's defective. If nothing changes, then you have a logic board issue which will require a micro-soldering repair.