high rev between shifts

well the prob with my dads truck ( 98 GMC half ton..4by...Vortech 350/5 speed) is this...frequently...the motor will rev up to around 2000 rpm's or so between shifts....it doesnt do it when first fired up....just after you start driving....he's had it in the shop and so far ....no problems can be found....this isnt a major problem...just really annoying....anyone have any idea what might be a good fix?? Thanks in advance