4kq exhaust questions

[Dan R]

When I first got my car (a '85 4kq) it had broken exhaust manifold studs.  I recently got them drilled out and fixed and got the exhaust fully attached again.  Less than a week after that the exhaust started leaking out of the joint just in front of the converter.  I took it to Midas and they installed a new gasket.  A few days later the same sound came back, and exhaust was leaking from the same spot.  I took it back to Midas today and they found that one of the bolts connecting the front of converter to the exhaust had broken.  They replaced it and said everything should be fine.  The problem is that now my car sounds worse than ever.  If I'm at high RPMs and I let off the gas then I will hear several short muffled thud or bang noise from the rear of the car.  It doesn't really sound like a backfire, but I'm not sure what it is.  This also happens between shifting gears at higher RPMs.  
Also at normal driving speeds the exhaust sounds louder now than it ever did, and I think this is the first time that all the exhaust has actually made it all the way through.  (The pipes are fine, no rust or holes or anything) It doesnt make sense to me.
 
Also, I let the car idle and held my hand behind the tailpipe.  The exhaust didn't flow out smoothly, there would be only a small amount of air flowing out, and every now and then a bigger gust comes through (every few seconds).  I honestly don't know if that is normal.

Is it possible that my converter is somewhat clogged or some other part of my exhaust is?  Should I ask Midas to test that?  What should I check?
I know very little about this subject, if anyone has info that may help me or if you need me to provide more information let me know.  Any advise is appreciated.

-Dan R.

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