I had the same problem as Lola. I had been 3 times prior, 1st visit was 1 month, 2nd was 4 months, and 3rd was 6 months. The 4th trip was only 5 weeks. On my 3rd visit, I got hassled as well. I even had an employment letter on that one since it was a longer visit. I was told to get married, and there was nothing stopping me from getting a fiance visa, and that next time there would be a hell of a lot more questions. And there definitely was!
The 4th visit, I didn't bring an employment letter (strongly suggest having one), didn't think it was needed for a 5 week stay. I did bring bank statements because on the last visit I didn't have one and was asked a lot of questions about my finances. I was detained for 4.5 hours. They had gone through all my stuff. I was asked a lot of questions of my family. Why do they live in one state and I live in another, when, and why did I move. They found a reciept I had from a car rental weeks before my trip to the UK (it was in my wallet). Whatever you do, leave all your receipts at home! I got asked a lot of questions about that. I even got asked about a trip I took to Australia I had taken over 4 years ago, who did I go with, why did I go there, for how long was I there, how did I pay for it, how could I have afforded it, etc. Very nosey.
Then they spoke with my fiance (boyfriend at the time) who was there to pick me up. We were repeatedly asked the details of our relationship. He said he may propose to me, and I had said, at some point I'm hoping he does. That was the wrong thing to say (but it was the truth) I was denied entry as a visitor, but was allowed a 5 week temp stay, which was just my return ticket. They kept my passport to make sure I did show up to the airport for my return. I was pretty much told, not to return unless I had a fiance visa (even though at the time we weren't engaged, and we had no intentions of being married in 6 months).
I was recently told, that you are actually only allowed a combined visit of 6 months in a 12 month visit, according to Home Office. It's not 6 months per visit, like we had assumed. If your combined is more then 6 months in a 12 month period then more questions will be asked by the immigration officer. It's not an automatic refusal, but the decision is up to the officer.
We did contact an immigration lawyer after the denied entry. The lawyer did make a point in...if you're not getting married (especially in 6 months), you do have the proof of means to support yourself, so what does that make you other then a visitor? Who knows. However, he did say, for visits like like ours, to have a visit visa before entering the UK, even though it's not required for a US citizen. It doesn't guarantee entry, but it may help with the 20 questions.
I think it also depends on who you get. I over heard the the entry officer (the one I had) speaking to another in the hall, and the other one didn't think it was a big deal for me to be there, at least not as a big of a deal as the entry officer I had was making it out to be.