No need for a splitter as I understand it. If there was a splitter in place with an unused port (telephone service) that splitter should be removed as being unnecessary/redundant and a potential source of the problem.
Revised line diagram:
ISP----> box(?) on wall --- Telephone cable ---- >Actiontec ---Ethernet cable --->ERX[eth1] ----- Ethernet-----> Gaming PC and
[eth4]-----PoE-------> Asus N66U ~~~~ wireless ~~~> Wireless devices.
"box(?) on wall" - Likely a RJ11 telephone jack. Any markings on the box?
In any case, there may be a loose connection within the jack or perhaps a problem with the cable run serving the jack or the cable between jack and Actiontec.
Take a look at the incoming wiring, the connections, and jacks. Look for loose connections and signs of damage. Swap in a known working telephone cable between the box in and the Actiontec.
Not disregarding some possible overload or other misconfiguration per se.
Overall idea being to ensure that there are no physical cable or connection issues contributing to the problem.