2 out of 4 Luminas that I have '90-'93 w/3.1s, have had the "putt"@ idle problem. The '93 had that from the day it was new (driven from dealership lot), which I learned from the person that I bought it from who was the original purchaser. It died about 1.5 years back @ approx. 380,000 miles.
The '91 that I drive now also has the "putt" and is still going strong @ about 250,000 miles.
I wouldn't worry too much about it.