Processing Wifi Packets
At the very least, you'll need a sampling rate that's high enough to capture the full bandwidth of your Wifi transmission. In a typical WiFi receiver, that puts the sample rate at exactly the OFDM width: 5, 10, 20 or 40 MHz, depending on your Wifi.
Your dropped samples simply happen because your computer isn't fast enough to keep up with that many samples per second. There's only two solutions to that:

Get a faster computer
Make your computer do less per sample.

Also note that the sustained write rates of permanent storage devices can be lower than your sample data rate, so even just writing to disk can be a fatal bottleneck.
Sadly, we don't know the processing you're currently doing, but maybe have a look at gr-ieee802-11, which is optimal enough that a modern midrange laptop can receive 20 MHz wide Wifi with it, continuously.