What is the make & model of both the SSD and the RAM you installed?
What version of macOS?
Do you have the 13" model and did you replace the hard drive SATA Cable? The hard drive SATA Cable has an extremely high rate of failure with the 13" model especially when using an SSD. You can test if this cable is bad by removing the SSD and booting the SSD externally using a USB3 to SATA Adapter, drive dock, or enclosure. If you don't have the issue with the SSD booting externally, then the internal cable may be bad or you have a compatibility issue with the SSD.
How did you set up the SSD? Did you install macOS, or restore from a backup, or cloned the original drive (what app was used)?
Did you enable TRIM?
To look for possible software issues run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.