Carnival crisis

FOR the past year my 2001 Kia Carnival automatic has had serious performance problems. Sometimes it loses power abruptly, at others it does not produce the power it should. I have taken it to an auto-transmission specialist, but apparently nothing came up on the computer. I also took it to my mechanic and, though I paid more than $900, the fault could not be fixed. I don't like travelling too far because of it. Could it be a fuel or transmission problem?