1994 Mazda Truck Idle while coasting

I drive a mazda b3000 pick up with manual transmission, with roughly 80k miles. Recently I've been having a problem which occurs every time i drive it. It will start out at a higher idle of around 1100 used to be 800, and when i start driving and as long as I am keeping rpms below 3000 when shifting which is fine on side roads but not on the highway when i have to get through the gears faster, after hitting 2nd gear and about 35 mph, sometimes before when i go to put the car into neutral and come to a stop the rpms as opposed to returning to 800 as before or the 1100 I am starting with will decrease to 2000 or sometimes increase to almost 4000 rpms and remain there while i come to a stop, sometimes it seems as though pushing on the brake makes the rpms rise faster.

Any answers would be greatly appreciated