Given the age did you do basic things like new spark plug wires and get the engine and compartment clean (dirt on wires can cause odd issues like this.) Also fresh plugs.
I have a seat ibiza 6k1 from 2000 with an engine of 1.6l.
I have the following problem:
When i start the car, everything is fine. The idle is running well with cold engine but when it gets warmed up the idle is fluctuating if I break the car. As I tried to inspect when its doing it, I realized that it happens only if I use the normal brakes. I tried to stop the car with hand brakes and it seems fine.
Other inspection was that when I slow down with engine break (not pushing any breaks, just leave the car in gears), there is an explaining sound coming out from the exhaust system repeatedly, but not continually. Also my car seems to be a bit noisier from the exhaust than before. (Not sure, maybe I'm just too suspicious)
Do you have any idea what the problem could be?
P.s. sorry for my English, it is not my native language
Thank you in advance for any suggestions :)))