Uneven idle and car stalls when slowing down or stop. What is the problem?

There are a couple things that will cause this but since you just performed a service, we need to start with two things. First the fuel pressure and second is the oil level.

Here is a guide that will help with this issue in general:

https://www.2carpros.com/articles/stall-at-idle

Here are a couple guides that will help with fuel pressure as well:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Lastly if these check out ok then we need to check for the engine for a vacuum leak. This will also be causing the issue you are seeing so here is a guide that will help with this as well:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Please run through this info and let us know what questions you have. Thanks