idle problem

[WILDACEX187]

i notice that on my de engine after i warm it up the revs drop between 300-500 rpms and then go back up. this also happens if i say rev it to like 2k rpms. they will drop and go back up again and then down and up. another thing i noticed that could be related to this is that when i rev high up in the rpm range and shift through the gears then when i start slowing down and put it in neutral my rpms drop really really low to like 100 rpms and sumtimes it shuts off on me and turns back on again. wat can cause this im afraid that it'll turn off on me on the highway and something might happen?