When you are using wireless do you sit in another room? If so what is between you and that room, you must consider all electrical devices.

Do you have a cordless phone? What other devices do you have?

May seem extreme but try turning all other electrical devices off to see if you continue to get the same problem.

All part of the ball ache process that is troubleshooting.

Don't just turn them off and expect an immediate solution, turn all off, reboot laptop/router and try sitting right next to the router, then move to other rooms/areas of the house.