With the new poppet assy and stats not fixing the problem, I would suspect the impeller also, maybe not reassembled correct, or bad base gasket,pilot tube and gasket not sealing or blockage (from old impeller coming apart?) in water tube going into the block. If you are getting hot enough for it to slow down, you are asking for trouble.Take a look at this article w/pictures. A little dish soap makes the impeller go in easier.
http://www.in-depthangling.com/forum...5/o/93/fpart/1