I too had same problem with the WLAN speed, always fluctuating from 5.5 mbps to 72 mbps.
I got my Dell Inspiron 14R 5420 4 months back.
I am using Windows 7 Ultimate 64bit . I have the same WLAN card (Dell Wireless 1704 802.11 b/g/n 2.4Ghz)
I tried many things to make the WLAN speed stable. I tried several versions of driver. But nothing become fruitful until I found out a simple solution that worked for me.
Just Follow these Steps:
1. Go to the Device Manager
2. Go to the Network Adapters and expand it. You will see your WLAN Card name there. (Dell Wireless 1704 802.11 b/g/n 2.4Ghz)
3. Right click the WLAN name and select PROPERTIES.
4. Go to ADVANCED tab and Select BANDWIDTH CAPABILITY from property.
5. Select the 11b/g:20/40 Mhz and Click OK (here the default value is set to 11b/g:20 Mhz which has been lowering the speed to 72mbps)
6. Now the WLAN had been set to increased bandwidth capacity. Check the speed from Network and Sharing Center. You'll be getting 150mbps WLAN speed.
This completely solved my problem. Hope this works for all having the same issue.
The fluctuation in speed still occurs but only if you are far from your router or if there are obstacles in between your laptop and router. Otherwise speed is stable at 150mbps.