One thing you can do, swap out xbox places to see if you notice if the problem follows. Swap cabling as well.
Another think you can check for on the ISP side of things, make sure that the cable lines are in good condition going into the ISP modem. Check for any t.v. line splitters. Remove them if you can. If one is needed, use a good quality line splitter. Dedicated cable line is preferred for Internet if possible. Have the ISP check the signal from there box to the modem. I would ask a level 2 or higher to make sure they are giving you accurate reports for signal. Level 1 will probably always tell you, "oh ya, signal looks good." Ya right. Demand a on site inspection if needed. If the signal isn't right for the ISP modem then this can cause a lot of problems that isn't router related.