Unit keeps shutting down

Hello,Hopefully someone can help.My microwave was shutting down constantly after running for a couple of seconds.I could reset the clock and try again but the same thing would happen.I posted those symtoms on here and someone suggested that I needed new smart board.So I got one and installed it.It worked for a couple of times and now I am back to the same problem.Do I have a bad board or is something else causing this problem or cuasing the boards to go bad.Please help.Thank you.

Do you have a meter? Can you do some electrical testing?

Update.I had read in a post somewhere of someone having a similar problem.And the expert said that the magnatron was going bad and interupting the signal to the the smartboard.So I tried a couple of experiments.I opened the control panel up and ran the microwave with the control panel open.First popcorn 2 and a half minutes.No problems.Then sausage 2 and a half mins.No problem.Then boiled water 10 mins no problems.So I closed the control panel up and tried to boil water again.10 mins with no problems.The only thing still not on the microwave is the top vent piece and I cant believe that it would be causing the problem.So now it seems it is intermitent.Any good ideas?Thanks

Still can be interference. Microwaves bounce off metal. There can be a "leak" in the waveguide, allowing the waves to bounce off other metal components. It’s real hard to tell without a microwave leak detector. Go ahead and replace the remaining part and see if the problem arises again.

Is there no other way to diagnose the problem other than a leak detector.I would hate to keep throwing parts at this if its not going to fix the problem.
Also,If I do replace the magnetron,do I have to dismount the microwave and disassemble it to do it.Thanks

The aim of my first response to your post was to say that you should test for 120VAC at the primary winding of the power transformer. The ideal test condition is when it is in a cook mode, and NOT working, as in your description of the original problem.
No power to the transformer would indicate a bad control board.