The comment by Brian from AD about not being able to restart a 2715E after air ingestion without bleeding is spot on. When either of my 2715Es stopped from air ingestion, it was a very sudden 3-4 second decline from cruise RPM to nada, with NO preamble, just dead. And there was NO restarting them until I turned on the electric priming pump and cracked the bleed screw on the Simms. Hint. get a pump installed before the new Racor 500s you are gonna buy.
BTW, one of my air ingestion points was a crimped o-ring under the tee-handle of a Racor 500.
Soooo, if you are able to restart at all without any bleeding of the system, the stoppage is likely not air at the injector pump, but as I think Ted stated more like a fuel blockage issue. Had a friend whose single engine liked to quit and would then restart. FINALLY he opened the fuel tank and found a piece of rubber hanging about near the tank's outlet. As soon as the loss of suction from the engine let this pesky thing float off an inch or so, the fuel supply was reestablished and he could restart.
So do you have fuel from multiple tanks lined up? This would eliminate this intermittent blockage at a single tank from stopping your engine. Are your tanks standpipe or bottom-fed? Standpipes can get pinholes in them, but then again that would allow air ingestion, which from Brian's comment seems to eliminate that as a cause of your stoppages.
BTW, if you elect to go the clear hose route to investigate, it takes a while of CAREFULLY observing the tube to detect any tiny stream of air bubbles - probably not worth the effort, yet.
On the good side, your engine sounds to be in great running condition, and I am sure IT is not stopping through some unique or odd mechanical issue. BTW, are you changing the Simms lube oil every 50 hours per the manual?
Suggestion: get rid of all the copper fuel line you can and replace with hose barb and hose connections to that new Racor 500.