I can't fix you shit dude, or your ISPs shit. maybe look at pysical issues first. Cabling wear and tear, using known good test cat 5 cables. Use surge protectors on your equipment, PS4 and modem. Resat all your cables, make sure there isn;t dust in the ethernet ports, could be that the cable is too near a power source and your getting cross cut interference across your cable. Could be your console ethernet port has craped out, same could be true of your modem port. Maybe your modem has crapped out, or your ISP used the cheapest possible one and is just shit in general? Could be that your ISP is blocking the ports Destiny needs to run on? maybe a cold reboot of the router? or the console ? maybe clear the cache of the console? Point is it could be a lot of things. Troubleshooting is never easy and it will take time to narrow down any issues especially if they are intermittent.