Does str mean rpm?
Smell of gas normally means you're running rich.
If your o2 sensor (lamda sensor) are older than 100,000 miles then it would be a good idea to do replace it.
Even if that isn't the problem, it's still good preventive maintenance... like changing the oil.
I would also check for vacuum leaks.
A dirty or bad MAF (mass air flow) sensor could also contribute to rises and drops in RPMs.
Idle Air Control Valve could also be bad.
Don't wait around too long to fix this problem. Take it to a shop if you have to. A car that is running too rich will prematurely wear out a cat converter.