LG G3 Signal on and off every few seconds after screen replacement

Noam Cohen , If phone was working with the old display perhaps try reinstalling it to see if you purchased a faulty display. If not take the phone back apart but this time clean all the connections(Qtip dampened with 90% or better Isopropyl Alcohol) check all pins for damage and straighten very carefully with tooth pick and reseat connections securely. Check the board for loose/damaged/burnt components and repair as required.

Check the battery with DVOM to see if it is taking charge, reassemble the phone and try. Good luck.

I hope this helped you out, if so let me know by pressing the helpful button.