Car makes weird noise starting + won't start at all at times. Suggestions?

[Lordblood]

I changed the O2 sensors (all 4) a few weeks ago due to the car not starting at all for a few days. After the change all seemed well, the engine started every time, there was an noticeable increase in power and power delivery became much smoother.

However, last weekend it stopped again, luckily in my garage after I had just started it. The engine stopped as soon as I let it idle and then wouldn't stay started (it would ignite but as soon as the RPM needle hit 800 it would stop again).

Now it is starting again and running smooth. However when I let it sit for a long time (more than say 5 hours) and then start it, the engine ignites and right when the RPM's go high (right as it starts) there is this screechy noise, almost like a belt slipping.

Any idea what this may be? Someone told me to check the throttle body but I want to see if there are other ideas.