Did you check if operating system did start saving RAM to the swap file frequently like in the early days of windows 95? Than you did run out of physical memory.
Turn all other software off before starting Shotcut and close and open Shotcut frequently while editing (especially after adding or deleting big parts).
Try to kill unwanted programs or unneeded processes with the task manager before editing. Maybe you have Windows and the RAM consumption of the OS did also grow over time!
The official FAQ says:
What are the minimum system requirements?
Operating system: 64-bit Windows 10 - 11, Apple macOS 11 - 14., or 64-bit Linux with at least glibc 2.31.
CPU: x86-64 Intel or AMD; at least one 2 GHz core for SD, 4 cores for HD, and 8 cores for 4K.
GPU: OpenGL 2.0 that works correctly and is compatible on Linux, Direct3D 11 on Windows.
RAM: At least 4 GB for SD, 8 GB for HD, and 16 GB for 4K.
NOTE: If your computer is at the lower end of CPU and RAM requirements, you should use both the Preview Scaling and Proxy features to help reduce preview lag.
Hard drive: yes, get one; the bigger, the better 