1.) Adjust your distributor timing and go for a spin. Adjust the other way round if it gets worse. If u're running points, get the gap right. This also involves trial & error

2.) Change the fuel filter.

3.) Replace the air filter or try running without the air filter for a moment.

But im pretty sure its related to no.1 since u mentioned high rev is ok. If your high rev is bad, then all 3 suggestions above need to be looked into.... :)