Having palpitations after delivery. Thyroid level fluctuating with medications. Less after stopping medication. What to do?

I been having palpitations since I had my second son. One time when my son was like 6 month old i had like a panick attack when my heart was beating very fast and went to the ER. They did a few test and nothing came back. I went to my donctor and he found out I had low thyroid . He prescribed me Levothyroxine and I thougt that was going to take away those palpitations. Well it did for a while but they came back and since then I ve always had those almost everyday and i have been to doctors and doctors and they say there is nothing wrong with my heart. Recently I went to have blood test done and dr said I had overactive thyroid and I had to cut back my medication but when they did blood test after 6 weeks it was low again. I dont understand. About a week ago I stop taking the Syntroid im taking now and Palpitation went away about 80% but I still get them sometimes in the morning but dont get them as often as before. Please help me. I have 3 kids and I want to be ok for them and this is stressing me too much and makes me feel bad and very unconfortable.