I've always had this problem ever since I started doing creme patisserie for souffles. After researching this issue in cookbooks, I found that I whisked the custard too much after it has been cooled thus breaking the starch cell and causing it to thin out (goes from dollop-y thick to watery sauce within seconds).
My understanding was that this could only happen if you whisk the starch-thickened things once they completely cool down; however, I had the same exact problem today except that i was working with a hot lemon curd (that was still on the stove). I have been making the same lemon curd over 10 years now, and it was the first time this ever happened to me (this was the first time i started using a whisk also...i figured that a vigorous whisk (over a wooden spoon) would prevent the egg yolks from curdling on the stove.
This is just driving me crazy, and I'm wondering if anyone experiences the same thing or has suggestions. I know it stems from over-mixing (mostly), but is there a way to prevent this? Temperature? Sans whisk? Potato starch?