Thank you for posting your question here on Fixya.com.
That is the diverter plate and it should be welded into the header. It helps direct some of the water flowing thru into the heat exchanger to heat the water. If the weld fails the diverter can interfere with that flow and cause overheating or poor flow thru the heater which in turn can lead to the problems you describe.
Remove the inlet side hose once you have blocked the flow of water from the pool and you should see the diverter in the inlet. If you can move it then it is broken and needs to be fixed back in place or the whole heat exchanger needs to be replaced.
If you can weld then tack in back in place. If not a local rediator repair shop MIGHT be able to fix it or else you need to replace the whole heat exchanger assembly.
If this answer does not fix your problem, please comment with additional details prior to rating the answer. You may also contact me thru our website at www.arrowpools.net. Positive feedback is appreciated once your problem is solved!
John
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