Some of these blocks tend to build up hard deposits in the water jacket at the rear of the block. Is it possible that yours may have this issue resulting in higher temperatures in the area of the sender causing the problem. I think my aftermarket gauge is Counterpart. I purchased it 8-10 years ago. The truck has been on the road for 6 years since restoration and it still works perfectly. The 2 aftermarket oil pressure gauges in my truck are a different story.
After my 235 block was machined I always go thru it making sure all threads and passages are clean and free of debris. No water would come out of the block drain at the rear of the block. Some incredibly hard crap blocked much of that rear area. It took quite a bit of effort and prodding with various wires and tools to eventually clear it. I suspect the PO may have repeatedly treated the leaking radiator with some sort of stop leak. In my case, had I left that in there #6 cylinder would undoubtedly run hotter than the others.
