There are three components that can vary the acceleration .. as you said is the iac valve the first suspect. It can also be the lambda probe of the exhaust .. and the flowmeter .. if it is a MAP or MAF air sensor it is easy to clean .. the first thing would be to clean the air sensors .. as you said a rainy day began to fail most likely it is a maf sensor and that it has had dust on it and that sensor has been covered with moisture. If cleaning does not normalize then there would enter a multimeter to dispel all doubts. I hope it helped you.