The other problem could be something to do with your antennas not matching on your quad and goggles as once could be right handed and other left handed which results in very poor range. Possibly even the location you flying has bad interference from other equipment such as power lines. Another common mistake is mismatching SMA and RP-SMA connectors, where the metal pin does not properly connect with the female connector.
Before your next flight after changing something its a good idea to try a range check by walking far away from your quad with everything on to see how far you can go, that way if the signal breaks down then you will not crash.
If it is an LC filter issue its usually noticeable only when you turn on the motors, so if you are getting bad inteference with your motors off then the LC filter is not the problem.
Otherwise I have written a coudple of FPV guides:
That should cover all the basics you need to know such as frequency and power levels etc... If you have any specific questions just let me know on the relevant guide and will be happy to explain.