Rough ride on specific RPM-range (5S-FE)

I’ve notice some jerking on 1800-2000RPM range. The engine runs smooth on idle and it mostly runs smooth on higher RPMs, but there’s noticeable jerking/tearing on this exact range. Could it be timing belt (I plan to replace it ASAP, I think it is 10 years old now), or perhaps it could be linked to IAC valve? Or could it be something else?

I appreciate and ideas that could point me in the right direction.