I think studying each component of a system to determine why it may be behaving in a certain way is the epitome of scientific; perhaps you intended 'technical'...
To examine an access point's or router's packet counters and stats, I suggest investigating [SNMP]. If your router/wifi AP supports it, it can provide an abundant account of interface attributes to assist in diagnosing a problem.1
Are all hosts connecting with 802.11n (or better), or is there a mixture? If you've no .11g devices, setting your wifi AP to exclude .11a/b/g will improve performance. There are a plethora of online WiFi tuning articles to be found to suit the spectrum of network configurations.
In this case, though, as stated, it's not clear if there is a wifi issue. This should be verified by transferring data between two hosts, and noting its characteristics: transfer rates, error rates, hangs, signal levels.
Concurrently, when experiencing performance degradation, verify that a wired host has good connectivity, which will also help identify whether WiFi is an issue.
1It depend on where and how you can tap in network. If Your WI-FI access point is combined with router? Maybe Your router/AP can show bandwidht usage by users? To run sniffer on wireless directly You need special Wireless hardware (Wi-Fi adapter with driver that give ability to capture all packets in air). – Mikhail Moskalev – 2014-05-18T12:36:38.493
What technology are you using exactly? – Max Ried – 2014-05-18T19:32:43.997
When you say "What technology are you using exactly?" - am i correct in thinking your referring to router / modem, rather than devices. We are using a BT 2wire 2700hgv (which is supplied by the network BT) - although ive also got a netgear range max dgnd3300v2 which i could use if it helped – sam – 2014-05-19T08:46:34.143