Overheating issues

The lower hose collapsing shows no spring in the lower hose.
During higher rpm, as the pump creates greater low pressure on the suction side of the pump, (try saying that twice), it can suck that lower hose shut, as you saw.
I use a chunk of stainless welding rod, or heavy piano wire to make a lower "spring/coil".
Rev it up higher rpm while hot, and check it out, might collapse right before your eyes .
Again, lol, good luck .