Common reasons why your internet speed may fluctuate include:

1. Issues with your modem or router.
2. Overheated devices
3. Using too many devices simultaneously
4. Network congestion
5. Faulty cables or connections
6. Software glitches

To improve your internet speed, you should:

1. Check your router/modem and replace faulty cables/connections
2. Make sure your device is not overheating and is in an open and cool environment
3. Limit the number of devices using the same network
4. Check for software or firmware updates on your modem/router
5. Contact your Internet Service Provider for any issues