2001 Echo 300,000 kms works great, smooth and quiet when cold, stalling after warmup.

2001 echo 300,000 kms works great, smooth and quiet when cold, stalling after warm up as though idle needs adjusting. Daily driver, many kilometers, always worked great with no issues. This change is sudden. Son has been putting premium gas into it lately could this be the catalyst?

The only other change was a clutch was recently installed 5 weeks ago. About 3 to 4 weeks prior to current problem.

Also there may be a minor exhaust leak and muffler is from another vehicle but the car still runs reasonably quiet.