I Was Put On Metoprol For Sinus Tachycardia. I Have

I was put on Metoprol for sinus tachycardia. I have had a stress test and echocardiogram and have also worn a holtler monitor. They say my heart is fine it just randomly beats fast over 140. When this happens I get dizzy and flushed. The metoprol is lowering my blood pressure to 99/64 which makes me feel horrible but I still have the random rapid heart beat. What would cause this?