"on the second visit I was given a very hostile reception by immigration"

In other words, the immigration officer did his/her job and questioned why you were traveling in an unusual pattern? They were doing their job. Provided that you have a valid reason to be there and can show this, then you shouldn't worry about it. They very well may question you, but that is their job. You shouldn't take offense to that and it is not a hostility. They do catch people doing things they shouldn't occasionally you know.

I travelled into the US [SFO] at the end of November last year and re-entered in the middle of December [SFO] ... no problems encountered at all.

On my second visit, the immigration officer asked me when was the last time I was in the US .. I gave him the exact date in November. He asked me how long will I be staying this time and I answered truthfully ie 3 days. No further questions and he stamped my passport.

:D

I have had some very odd questions from immigration officials, but the key is to be honest and stick to that story, half of the questioning seems to be checking for self-consistency.

I've previously been hassled for not knowing the dates of my last visit (at the time, I was travelling abroad weekly - sorry, but as nice a place as the US is, it's not important enough for me to memorise dates), and, the most bizarre of all, for "spending so much time out of the office I can't possibly get any work done".

I have also no problems visiting US twice in two weeks. I had this last time flying from Europe to LAX - first immigration - 2 days later to AKL and 5 days back to LAX - second immigration.