Are you playing the music 'off line'? If so try turning 'off' your phone's internet connection and the WiFi as well.
This will enable most of the phone's resources to go to the Bluetooth service. See if this improves the connection.
Go to Menu > Settings > Wireless and Networks > more > mobile networks > data connection. Select off. This setting only turns off your phone's data connection. The phone still works as a phone but you may not correctly receive multimedia sms' (mms ), ordinary text messages (sms) are OK and of course the internet does not work. Remember to turn the setting back to on when you want to use the internet or receive mms'.
To switch off the WiFi , go to Menu > Settings > Wireless and Networks > WiFi > swipe button left to off
If you are streaming music, then maybe you should check what other apps may be using the internet at the time and see if you can prevent them from doing so. Be careful not to turn off any system apps. If in doubt, don't.
Also, how far away from the phone is the Bluetooth speaker? Most of the common Bluetooth devices don't have a range beyond 10 metres (33 feet). Are you operating it within this range? If you are at the limit try reducing the distance between them. Also try it with the charger connected to the phone and on, just in case the phone's battery is performing below its' best.