This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
Problem: on 84 Laser, 88 Omni, and 87 Shadow (2.2 liter, turbo and
non-turbo), idle suddenly jumps from 800 rpm to 3000 rpm. Sometimes goes
away quickly, sometimes doesn't. Solution: turning off the defroster; check
the speed/distance sensor and connection (Tom), freon level in the a/c
(james eldridge), and the wiring harness on the back side of the engine
(Jeffrey Wieland). Jeff found that the wiring harness got hot enough to
damage the wire insulation; he spearated and re-insulated the wires, which
fixed the problem.
High idle, 2.2/2.5 TBI:
Most likely automatic idle speed system. Check for fault codes. Check
wiring harness near AIS motor for shorted wires or wires that seem stuck
together (separate and insulate from each other). Also check EGR system,
vacuum system, and timing.