I was told if the problem is on the bottom of what you are sewing, the solution is the top thread.....
are you sure you have the top thread, threaded correctly ?????
when was the last time you changed your needle ????
I have had problem with sewing, and discovered (more than once) that my thread coming off my spool was getting caught in the little cut that holds the end of the thread.....I try now to put that side of the spool in first (and no I don't always remember, but I try.....
If all else fails, CALL your dealer...ask what they did for $80.....they SHOULD be able to walk you thru a solution.....
and may I strongly recommend you insist on sitting down and test driving your machine after service in the future....
I had a very similiar problem.....had to make another trip to my dealer (over 100 miles away) sat and waited for the repair and then did a test drive on machine.....the only way I will do it in the future..... and that learning curve was only fifty years in the making.....