some stuff to try:
1. is your power cable secure? during a fight is the extra movement cause by ACM moving the stick and cable and causeing power loss to the stick i.e. FF stops working.
2. I also have a MS FF2 and when the sensor on the grip gets gunked up it just means the FF is permenent and doesn't need my hand in the way to activate it, so I dont think that it would help to clean it but it would be more pleasant

if its usb model:
3. Check USB ports i.e try another
4. Full uninstall and reinstall of MS FF2 software
5. update usb chipset drivers
6.Full uninstall and reinstall of all usb devices and then install MS FF2 stick first to make sure usb bandwidth is allocated properly.
hope these help.