Are the 802.11n devices also connecting on 2,4 ghz or are they using 5ghz? Then perhaps your issue is a noisy 2,4ghz environment. Try verifying your radio planning with placements/shut down of some 2,4ghz radios and lowering of Tx power to minimize co-channel interference.
Also updating the client drivers is a good thing to do to see if it brings any improvements.
Cheers,