I am no expert on X-plane, however, there are others on this forum that have and had the same problem.
Compare your computer specifications with the minimum and recommended specifications to run X-plane satisfactorily.
As mentioned by you and commented upon by others in other postings, your flight crashes after some flying time. If your "Mac is aged" it may not have sufficient cooling within the case. If your "computer" and "screen" are an integrated unit then that may also be part of the problem due to insufficient cooling. It is also worth considering the capacity of the computer's power supply ie is it a 350 watt unit or 650 watt plus unit. The higher the number the lesser of the risk of insufficient power to operate the computer.
Hope this helps.