Why my 2005 Sentra runs smooth in park/ neutral but a small rough idle in drive?

What is your question?

Car starts and runs ok and no check engine light is on. In park or neutral runs smooth but in drive have a small rough idle. Put new spark plugs in 3 months ago. Just replaced cadilitic converter on the car that was causing the car to run real rough and check engine light to come on. Each spark plug has it's own coil to it how can I tell if one of the coils is going bad? No check engine light is coming on now.

What makes this problem better or worse?

When the car is only in drive goes away in neutral or park.

How long has your 2005 Nissan Sentra had this problem?

around 3 months