nissan hardbody breaths

okay so my stepdad has a nissan hb, v6, se, 4x4 its iether a 94 or a 96, but wen hes at an idle, or is coasting, the truck breaths, by this i mean, the rpms go from 1 to 2.5, its done it ever since he got it, and we cant figure out why, its also really bad on gas i was wondering if ne 1 would have ne idea's of why it does this