I think my Mac baby is dying

I have done a little work on my MBP and it hrlped for awhile but now, 2 years later I am having new issues. The new issues are that apps are crashing and the hard drive is now inconsistently identified, at times the machine does not recognize a hard drive at all.

2 years ago I had some issues that began with the battery failing. I can use the machine plugged in but the battery wont charge. That and other issues led me to replace the harddrive, the darddrive cable, the sleep sensor & cable, the DC-in, the battery, the keyboard, upgrade the ram to 16, and I cleaned the old flux off the heat sink and applied a fresh coat.

For awhile I could tell a major difference in how well it functioned but it started slagging again. I tried upgrading the OS but it didnt help. Tried another hard drive, which was also not recognized.

What are your thoughts and advice? Please??? Am I better off buying a new logic board or just replacing the few parts that need replacing. I see no water damage, btw, though havent had it checked w/ a UV light. This laptop is on its way to becoming my “experimental” laptop but I would like some help to aim me in the right direction.

Ty!

Deborah