Couldn't one of the repair guys help you to be sure you have it threaded right? I don't know anything about this kind of machine, but if the repair guy didn't test it with the exact same set-up you're using, check anything that's different from what it had in the shop. One time my "tension issues" were actually caused by the thread catching on a damaged needle plate.
Am I seeing correctly in your pictures that there are some stitches that go along with no problem for several inches at a time? One other idea - really out of left field, but maybe not impossible - do you have flickering lights in your house? Slight disruptions in power could make the machine erratic. We used to own a house with aluminum wiring, built during the Vietnam war. Aluminum comes loose over the years, and the only solution was major $$$ electrical work. I hope you don't have that! If you suspect this, possibly get a heavy duty extension cord and plug it into an outlet on a different circuit. Even without aluminum wiring, it might be a good idea to get an electrician to make a dedicated circuit for this machine so that you don't have to compete for power with other items in the house. At least make sure it's not on the same circuit as the fridge, coffee pot, microwave, etc.. I just have a hunch that this industrial machine would draw more power than ordinary DSMs, and more than modern LAs.
I personally have no objection to reading a post like yours wherever it turns up.