The crew debriefings from other Apollo missions make it clear that the "jolt" at staging was normal. I've written IMDB pointing this out. The blooper page is the only place I've heard about the supposed early firing of the ullage engines.
On 2002-02-11 23:38, Peter B wrote:
I just had a look at the website of "Apollo 13" bloopers (surprisingly many!) and one in particular caught my attention.
The first stage separation shunt is described on the website as being totally unexpected, and caused by the ullage engines firing a second too early.
Everything I need to know I learned through Googling.