Idle problem with 86 5ktq

[Jim Herzig <jimzig@athenet.net>]

I ahve just begun to experience a problem with idle on my vehicle.  It
began after highway driving at 70 mph for 30 minutes.  When i pulled off
the exit and stopped at the light, the car would idle up and down between
1500 and 2500 rpms contiuously while I waited.  It has done this before and
stopped after i shut car off and restarted but the problem now continues
every time i drive the car.

Also, i noticed while city driving in 4th gear at 35 mph the car will jerk
forward when i give gas to keep up speed and jerks back again when i let
off the gas.  As i struggle to maintain the vehicle at a constant speed,
the car jerks back and forth depending upon whether i am giving gas or
letting off.

Any suggestions are much appreciated.