Theresa,
You mention loose belt sound, so try investigating "under the cover". You will need to undo the screws on the top cover , if you don't have the manual then take note of where the plastic shims are etc in relation to the screws you take out. The top cover has some electronics connected to it so you can rest it on top of the machine without completley disconnecting it. Check the motor brushes of the motor these are the black caps either side of the motor, once you undo these a spring will want to jump out so be ready for it. On the end of the spring are the brushes make note of their orientation. Measure how long these are, if it is less than 3/8" they need replacing. While you have the brushes out take your compressor place a cloth over the hole on the other side and blow it out. These may have worn out which leads to the motor slowing down.