Fill Not Reaching Perimeter

I have a problem with the fill not reaching the perimeter? it's not always the same perimeter but is very consistent when it happens. this problem occurs with solid fill or infill exactly the same. i have a reprappro mendel and am using the latest version of slic3r. attached is an example of the problem. ignore the quality of the print job on the product - it's crap because i was trying something to fix the issue and it didn't work. normally my prints are fine except for this edge problem which obviously creates a great weakness in the final product. you can see the gap (black) which contains no material where it should. i am probably just stupid and don't have something set right. frustrated in california. any help would ge GREATLY appreciated.
thanks
dennis

Somehow looks like having backlash on X and Y. Could you check that:
- belts are tighten and
- small gears are well attached on the motor shafts?

3dmaker4u-

it was the belts. not only did tightening the belts fix that problem but the fill pattern looks COMPLETELY different now that the printer is working right. the prints look much cleaner overall. this problem has built over the last year and it happened so slowly i didn't even notice it. i may now have the belts too tight now but i will watch for any issues. i guess the belts just stretched over time and with lots of use.
thanks VERY much.
dennis