Check to see whether there are holes where the stitches should be. If there are holes, it is probably a problem with your needle, bobbin, tension or machine timing.
If there truly are skipped or irregular stitches, make sure that if you have a stitch regulator that it is set to the unregulated mode. If you have an Intellistitch, also make sure that motion detection mode is off(one beep at startup). If you have two beeps at startup, hold down the black handle button at the back of the machine during startup to turn motion detection mode off.
Also, check your speed and detail settings on the IQ. If they are too far apart, or set too high, this can affect the stitching quality. Higher iQ speeds require higher stitching speed. The needle flexes more at high speeds, so if the hook timing is marginal, i.e. at the limit of being affected by needle flexing, skipped stitches will happen at these speed settings. Higher tension on the thread just makes it worse. Each pattern has its own ideal settings for speed and detail, but a good rule to follow for patterns with points and straight lines is to reduce the Motor Control Sensitivity to 14 in System->Interface Setup/Test and to use dwell 1.