once my 2000 cherokee sport is completely warm the oil pressure will drop to 0 unless engine is above 1000 rpm's

chek your oil pressure sending unit, usually located beside or near the oil filter. not sure about the 2000 yea though. that might be the problem. do a man ual test with a gauge, if it seems that the oil pressure is fine then its the sending unit. just replace, its super easy.