This question came to me after reading the thread started about Angelina Jolie playing Patricia Cornwell's Kay Scarpetta in a future movie based on Cornwell's books.
A few that come to mind for me are:
Dean Koontz -- I loved his books when I was much younger, but after the 450th alien and/or government conspiracy story, I felt his novels were becoming a tad formulaic

.
Stephen King-- He was always a little out there to me, but his older books, especially those under Richard Bachman were at least enjoyable.
Robin Cook -- I felt he too was becoming a little bogged down in formula.
Every now and then I will pick up a new release from one of these authors (or others I've liked in the past) and sadly it's like going back to an ex-boyfriend -- I quickly realize why we broke up in the first place!
Also, are there any suggestions for any new authors who might be pretty good? I do love a good mystery (I'm writing one of my own!) and I am starving for a fresh, compelling story to read.
And please, no flames! These are merely my thoughts about authors I thought wrote captivating stories in the past, but as the years went on, their books only got more strange or more unbelievable.