Timing belt issue roughly idle up and down?

Hello, I recently did a head gasket change and now I feel like I have timing issues. The car will start but sort of roughly idle up and down until it settles at about 600-700rpm. Then sometimes right as I tap the gas, the rpms drop before revving to normal. I thought maybe it was an air filter or spark plug issue. So I replaced both and when I pulled the old plugs they were matte black as if they were running rich. I'm not sure of the issue or if I did the timing correct. Thank you!