C10 Issues

[Simple10]

Hey guys. I hope someone can help me out here, I have a C 10 and when I put the fan switch on full the air conditioning works great, when I turn it to medium or low on the switch it still works but you can’t hear the fan and when I give it a boot full of acceleration when driving it feels like I’m running on 7 cylinders .....missing and farting , pig rooting , then I hit the Aircon fan back on high and it drives like a dream. any ideas where to start?
Must be an electrical problem somewhere .