Juki MO-654DE Mechanical Sewing Machine

I purchased the machine as refurbished from someone with excellent feedback who deals with sewing equipment, so I felt quite confident in my investment. When I got the machine it was threaded with the four thread double needle and had a sample under the presser foot of what the machine was capable of. Indeed when I tested it, it did exactly as shown in the sample. I decided to remove the second needle and go to a three thread stitch and it no longer worked as efficiently. Now it stitches along fine for a bit and all of the sudden it misses and then goes back to being fine then misses again. Rethreading doesn't work. I'm disappointed that I can't use it until I take it in for service. I should have purchased the new model instead of going for a used refurbished machine for a lower price. Shame on me for trying to save money because now I'm going to have to pay to have it serviced anyway.