rpm not staible when swich AC

Hello, you will have some minor idle fluctuation when the air conditioning compressor cycles. But it shouldn't be dramatic. If it is more than a like comparative vehicle, then I would focus attention on ensuring that the air conditioning system is not cycling too frequently, suggesting a possible lower than ideal refrigerant charge. It could also be related to some kind of gasoline engine drivability issue which needs specific troubleshooting. You cannot manipulate or reset or adjust idle speed, so you have to look at controlling factors if you intend to improve anything. But, in the end, the best thing to do first before you undertake all this is to compare your truck's performance with a like vehicle to see if there is a real difference worth pursuing. Good luck.