Using the upscaler with frame generation performs flawlessly with my RTX 4090 in standard GTA V. However, when running it alongside the LSPDFR mod, the results are inconsistent. Sometimes it crashes repeatedly, while other times it runs smoothly for a few hours. So, to directly answer your question—at least for DLSS—the answer is no; it's not entirely stable with LSPDFR.
One suggestion to improve performance, especially if you experience lag after extended playtime, is to increase your system's VRAM allocation. You can do this by navigating to Advanced System Settings in Windows > Performance. Adjusting this setting may help mitigate the performance issues.