1- Have a plumber Auger the toilet for a possible obstruction like “Q” tips
2- Recheck your vent
3- Pour 5 gallons of water directly into the bowl and look for a vortex
4- Reach under the bowl top (where the seat fits) and make sure the holes are completly open and not blocked with lime or other mineral deposits
( you can hold the bottom hole closed forcing the water to shoot out of the upper most holes)