Weird issue with tub overflow getting full of gunk

[NyCad]

I have this weird issue where my bathtub/shower combo overflow drain is getting coated with gunk and nasty slimy stuff every three weeks. Creating a nasty smell which comes from the overflow cover. I cannot tell what is causing the stuff to get in there. One thing I notice is that it is close to where the main drain meets with the p trap. Could excess water be building up and entering the overflow, allowing buildup of grease? My shower head has the flow restrictor broken and the water flows like Niagara Falls. Could the water be overwhelming the p trap, thus allowing water to rise up into the overflow pipe coating it with the grease? Please let me know what I should do as I have cleaned it 3 times every 3 weeks only for it to show up again with the smell and build up in the pipe. I can tell with the borescope I have.

Thanks