If you are sure that the problem is the number of connections to a destination IP address, you should try and implement proper QoS that equalizes clients bandwidth, no matter how many connections each client has. For more QoS info you can start reading:
http://forum.mikrotik.com/viewtopic.php ... 78#p141378 ;
http://forum.mikrotik.com/viewtopic.php ... 28#p137128 ;
http://forum.mikrotik.com/viewtopic.php?f=13&t=29211
But the problem could be a little different, as there are a lot of problems with the Wi-Fi itslef. So tell us more about what's happening, to look further into it.
Good luck.
P.S. Here's the
changelog (first post, attached .rar, open txt with WordPad) from ver 2.9 upto the latest release of ver. 2.9.51, if you find a feature that has been fixed that you are using, you should consider updating the RouterOS version.