Wungun RT

can anyone answer this question......my parents 90 integra gs automatic recently recieved a newer engine due to the original blowing.....everything was fine up til about 200 miles after the swap....the engine when started has a rough idle but normal rpm at start up, but as soon as you put it into gear it drops rpms like it wants to shut down....can anyone help solve this problem, i figured it to be the (fitv)and or the (iacv) but when checking them it did not solve the problem.....please help guys i would appreciate it....