As the regulars in this forum know, I have had a very similar problem with my 2001 LS for over a year now... For me, it is a very intermittant problem... The car will be perfect for several months and then out of the blue it happens again... I've changed the fuel filter, fuel pressure regulator with no luck... Next on my list are the cam and crankshaft position sensors and the fuel pump... By the way, your CAT replacement should have been covered by GM as it is a know defect and GM extended the warranty on it for 10 years, 120K miles... Good luck and keep us posted!