Do not use starting fluid. Period. Unless you like replacing head gaskets.
Don't ask me how I know.
Use WD-40, if you really need some boost.
You describe same problems I had with my 2QM15 when I just bough the boat.
In my case it was a problem with a fuel and fuel delivery system.
There is very simple test you can run. Warm up the engine.
Get a small funnel and a hose and attach it directly to the injector pump, by -passing all fuel system - tanks, pump, filters.
Just pour a quart of clean(!) fuel into funnel and let it run by gravity feeding. If engine works well (after initial coughing and splitting) then your problem is with dirty fuel, leaking connections or faulty lift pump. I would give 90% chance that it is a case.
BTW, I don't have shut off cable. Engine shuts by pulling gas cable all the way down.