Bad idle and does not want to run under 45 mph

Octane booster will make any spark-related problem worse. You want to use the lowest octane gas you can find. Higher octane gas makes it a lot harder to ignite. That is used to allow the engine designers to use a higher compression ratio to get more horsepower, but the drawback is greatly increased preignition, or spark knock. The higher octane reduces that spark knock, so that is only used when it has to be used. You will never solve a problem by switching to a higher octane rating. Many people incorrectly think the higher octane develops more power. In fact, all it does is allow the engine designers to design engines that can create more power

Everything you've described points to a plugged or collapsing fuel strainer inside the gas tank. It's shown in the photo. This happened to me twice on my '88 Grand Caravan and twice on two older cars with carburetors. The first time with my Caravan, it ran fine for over 200 miles, then stalled when I slowed down to turn off the highway. Later, it took me over four hours to get through Minneapolis with all three of their interstate bypasses down to one lane for road construction. Once back out on the highway, it ran fine the 200 miles back home until I slowed down to turn off again. The symptom was the engine stalled when the largest volume of gas was being pumped, which is during coasting.

Now that I know why this happened, there is something you can do to try to verify it. I never got the chance on my van, but I think this will work. When the problem occurs, unplug the vacuum hose from the fuel pressure regulator, (red arrow in the second photo), and plug the hose. If the engine runs okay now at all speeds, but with possibly a little black smoke from the tail pipe, suspect the strainer. Doing this creates the condition under which the lowest volume of fuel is being pumped, so it's easy to get the required volume through the partially-plugged strainer.