If you just ping a random wireless client, that is just "sitting there", it might show you a graph that looks like yours, because a lot of wireless clients do power save on their wireless NIC.
This means it signals the wireless network that it is now going to sleep, the AP buffers the traffic, and advertises the client (traditionally in beacons) that there is traffic for it and it should wake up.
The clients NIC then polls the AP for the buffered traffic.
This of course is something that happens very quickly, but it will show up in fx. continuous ping graphs.
Sometimes, but not always, depending on the client, OS, driver and so on, the wireless NIC might not go to sleep while connected to power.
If you experience trouble while just doing normal operation (work) on a client, then, as many has suggested, you start troubleshooting at layer 1 and do a wireless sitesurvey, to ensure that signal strength, interference, noise and so on, are ok, so that the client can get the signal it needs, in order to perform well for your applications.
/Thomas