Janome 350e embroidery machine will not sew well?

i have purchased a janome 350e embroidery machine as an add on to our garment printing service in the hope of more work but i get constantly bad results when doing polo< sweat or t shirt pockets the stitching is shabby uneven and often the shuttle thread shows through as well i have tried all manner of stabilizers inc iron on ,tear away,water soluble even 3 layers thick i have altered the tension but even when near 0 shuttle thread still comes through i have had the shop check it and they say it is ok anyone out there got one of these machines that can help at wits end now and ready for giving up if it was not for the money we have paid for it