Bootable but unstable — that's a failed RAM upgrade.

Random system crashes, blue screens with memory-related error codes (like MEMORY_MANAGEMENT or IRQL_NOT_LESS_OR_EQUAL), and frequent app crashes are the most common signs.

If you managed to boot but the system behaves erratically—freezes during normal tasks, fails to complete a memory test (MemTest86), or only detects half the expected RAM—the soldered upgrade didn’t take properly. The joints might be cold, bridged, or the chip itself is damaged.

Don’t trust it. A machine that boots but corrupts data silently is worse than one that won’t turn on at all. Run MemTest86 for at least one full pass before you call it good.

Buddy up with someone who has a hot air station before trying a rework, or accept the cost of a replacement board.