Engine "stumble" In N And P?

[K9crew]

My '99 LS has been trouble free for 20k miles (bought at 58K now at 79K) but in the past few weeks I've noticed a slight stumble in N or P but not in D. When I'm waiting at a light I usually put the transmission in N (sometimes P if I know it'll be a while) to reduce heat in the tranny. I've been experiencing an intermittent stumble in the engine when doing this. Does this sound like a vacuum leak? What's the best approach to finding a vacuum leak? What else could be the cause?

Thanks

:)

[K9crew]

BTT

BTW ... I changed plugs, fuel filter, air filter, pcv valve, oil and oil filter in the past few months.

:)