"Farage is becoming more popular because he is not being questioned on absolutely key issues." Totally agree. Farage is popular because he communicates well in a reasonably affable manner. He gives simplistic answers which are often misleading or untruthful, but all the same, he makes the public feel he is on top of the situation. He has persuaded a large number of people he knows his stuff and has pinpointed why things in this country have not gone as well as it should - the EU. His party has a clear, unambiguous message and Farage has been totally consistent about this.
He has stumbled into a political vacuum created by the two main parties which are hopelessly divided on the issue, but they think the public is unable to see this. Farage can't believe his luck and will take every opportunity to hammer home his message of opposition to Europe.
Unfortunately, he is on the crest of a populist wave which has created the Brexit issue, put Trump in the White House, has swept over much of Europe, India and most recently Australia.
You are right about the things he said in the past, eg the NHS should be replaced by a private insurance scheme, and these statements should be used to highlight that what he says today he will not be able to defend in a few years time and basically his political judgement is not to be trusted. But apart from Andrew Marr last week, I haven't seen many others make life uncomfortable for him in front of the cameras.