iwl5300 very slow with excessive Tx retries

My home network is hosted by hostapd (with HT enabled) on a Ubiquiti SR-71-E (supported by ath9k). My laptop (running Ubuntu 12.04 with kernel 3.2.0, I can try 3.4 soon) has an iwl5300 (8086:4235) and sadly performs quite poorly in transmission. Namely, a large transfer over SSH will run at around 1.5 MB/s with iwconfig showing the following,

wlan0 IEEE 802.11abgn ESSID:"LittleGreenMen"
Mode:Managed Frequency:2.422 GHz Access Point: 00:15:6D:85:22:E2
Bit Rate=39 Mb/s Tx-Power=15 dBm
Retry long limit:7 RTS thr:off Fragment thr:off
Power Management:off
Link Quality=55/70 Signal level=-55 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:21262 Invalid misc:63 Missed beacon:0

Note the high "Tx excessive retries" quantity, which increases at the rate of about 1000 per second during the transfer. The "Invalid misc" quantity also increases although at a much more modest rate.

A bit of Googling shows that I am not the first person to experience this issue, with others reporting improvement after disabling power management (iwconfig wlan0 power off) and disabling 11n (with the 11n_disable=1 module option). While the former changes nothing in my case, disabling 11n stops the runaway Tx retries, although "invalid misc" events remain and throughput does not improve.

Any ideas what might be causing this poor performance?

Cheers,

- Ben