It's hard to say for sure from here, but it sounds like something is slowing down your wire feed. Check the contact tip first to be sure it's not dirty enough to slow down the wire.
If you have not adjusted the settings on the machine, and the tip is good, then it's probably time to clean out your gun and liner. The inside of the liner and gooseneck on the gun gets stopped up with dust and metal shavings after a while. A quick way to clean it is to remove it from the machine, remove the electrode wire, and blow out the liner with compressed air. Also, check the lead for damage. If something sharp and heavy enough to cut the insulation has fallen on the lead, it may have kinked the liner. That would require liner replacement.
Also, when you're welding, try to keep the gun and lead as straight as possible in order to reduce liner friction. If your lead is coiled around and under your feet, it won't feed properly.
You should check the drive rolls as well while your cleaning up the machine. Make sure they are tensioned properly and not worn out. If the wire groove in the rolls is worn out, it won't matter how much tension you apply, they still won't feed right.
While you've got the compressed air out, use it to blow the dust from the machine as well. Open up the casing enough to get the air nozzle inside and blow it out good. This will help it run cooler and prevent electrical shorts caused by metal dust.
Now that your machine is all cleaned up and maintenanced, try it again. If you still have troubles, come back and explain in a little more detail about what's going on.
Good Luck!
Tim