1990 Celica GTS surging after warm up

we have a 1990 Celica with a 5fse engine that starts right up but after it warms up it surges between 2000 rpm and 800 rpm. The check engine light is NOT on but we get a code 24 when we checked it by grounding it. Will an air intake temp sensor cause the surging?? or should I look for some other cause? This is my son's car and I do not want to put very much $$ into it.
(this is his problem, but I will help him out a bit) any guidance would be appreceated.


The lakes will freeze soon and we will be icefishing again :)