AT&T U-verse → Att Uverse unreliable speeds

Hi,

I have been very dissatisfied with my att verse connection lately.
I pay for a uverse 3mb's a second plan(the fastest available in my area). Sometimes the connection will achieve its full speed of 3000 kbps and all is good but then it will drop to 765 kbps and is almost unusable. I know there are some issues on the line because it always tell's me there a device on my network without a filter(a device i cannot find). It also cannot pass the 6rd conunity test( motorola nvg510). Is there any way to fix this. Thanks.