rotarypowered29

Hey Everyone I have a concern with my 2004 RX-8 and if anybody has had this same problem or know of any suggestions I would greatly appreciate it. My rx8 about a month ago started cutting off on while at idle and when driving it. The Engine would just shut off. Took it to the dealership and did a compression check and they said i was loosing compression. I bought a used engine and had another mechanic install it because obviously the dealership wanted too much. After it was install it would start right.....I should mention that my other engine had major flooding issues but this one didnt. Now it wont Idle properly and it will fluxuate and then it will shut off.....I have tried driving it and just turning it on and off for several minutes and it still wont idle. I really need some help because they have done every check they could think of and still dont know what the problem is other than it being a faulty engine.....again. Thanks in advance