One of the most common complaints bringing customers into my shop is low power and/or an erratic powerband.
I'm very good at rapidly diagnosing and repairing the common (and relatively inexpensive) power robbing ailments.
Common causes of low performance are:
-Clogged or malfunctioning EGR & intake
-Leaking vacuum system
-Under-reporting air mass meter (MAF)
-Malfunctioning boost control devices