Suzuki Club UK

I have a 1998 Vitara 1.6, 8 valve on single point injection the car starts and runs just fine but at mid range on the pedal around 3000 revs it starts to holds back and becomes sluggish, but it will eventually drive through the rev range and then its power is just fine. It has recently just had a service including spark plugs and fuel filter but the problem is still there.
Has anyone had and cured such an issue or can point me in a direction of what could be the problem?
Thanks in anticipation