Welcome to the boards.
I think the dose increase was too much, you should increase slowly by 25 to 30mcg incriments, 75 is a big jump and can cause all sorts of symptoms. I would stay on the lower dose and speak to the dr first especially since you were originally on 100 not the 200 so it may be that he was thinking he was going down by 25 not up by 75.
Also, the TSH can be very deceiving, your dr really NEEDS to be testing and basing treatment on your FREE T3 and FREE T4 and since you have a heart related issue you really really NEED to have the FREE T3 checked, if it gets too high it can cause heart related symptoms and a jump in meds as high as you had may be what caused your episode of SVT.
Both your Free levels NEED to be in the upper 1/3 of the lab range, but if one is too low and one is too high it can cause symptoms also.
Please read all you can about your condition, the Informative Archives at the top of the posting page has a lot of very useful info in it, its a lot of reading, but well worth it.
Keep us posted.