my baby has about 180K miles on it and it’s a manual transmission. It’s a champ, and I love it. But sometimes when you first drive it ( not sure if it’s only when the weather is cold, but I don’t think so) if you are sitting at a stop with your foot on the brake then engine surges up and down, if you take your foot off the brake it stops.
We have had it in to the shop but of course it never does it for anyone there.
My first guess would be a leaky brake booster.
Not unusual on a 25+ year old car.
My 99 limited run idles great until the engine reaches the operating temp. (warmed up) It then idles very smooth for approx. 7 secs. and transitions (about 3 secs.) to a rough idle for approx. 7 secs. It continues this pattern for as long as the engine is at normal op temp. The transition period is approx. 3 secs. You could measure time, it’s that sequential. Any ideas are welcomed. It has been doing this for more that a year and very rarely will stall.