Also, I have found that these days, domestic sewing machine shops are populated with some pretty non-technical people. Even some of the technicians are non-technical. It's sad. If I were you, I'd see if I could find a shop that sells industrial machines. Call and get their opinion or have them look at the machine.
Also, a lot of times, these cleanings basically are about doing superficial things to make you feel you got something for your money. (No offense to repair people who do a better job, this is just what I found out. I had a machine "cleaned," opened it up and a giant dust bunny popped out. It hadn't even been opened -- for $100. That's when I started doing cleanings myself.) Anyway, don't figure that your machine really got cleaned (esp. from an org that doesn't know how to fix a bad presser foot). One thing to do is try and "floss the tension disks with some heavier thread to clean them of micro-lint that may be affecting tension.
And yes, rah-rah to those on the other forum who suggest Signature thread. I love that stuff. I also love Glide (which is a polyester).