what would cause the engine to have a low idle, then surge when driving hwy on 2002 Ford Ranger

it idles low when stopped, then when running at a higher speed it seems to surge. at times the brakes are hard to push.

Asked by for the 2002 Ford Ranger
Needs to be checked for correct intake mauifold vacuum. Then for vacuum leak(s)! Probably needs to go to the shop.