Microwave Power Low

I have a Frigidaire Professional Series over the range microwave (FPBM189KFC). A few months ago it quit heating. I replaced the magnetron with an OEM one along with the diode. The microwave heats now, but is not very powerful. I did a few tests and it looks like the estimated power is around 650 watts as opposed to the 1000 watts it is rated. Could the magnetron be bad or is there something else I should be looking at. I was really wanting to keep the microwave because it matches our other appliances. That is really the only reason I am repairing it versus replacing it. Any help or suggestions would be appreciated.