Well "at stock" doesn't magically grant complete stability. For example, do you know for a fact that this "original transcend pc 3200" is completely stable?
Until all parts of the machine (software[drivers, os] and hardware[psu, gpu, ram, cpu, mobo, etc]) are isolated and tested thoroughly you can never know if one of them is giving you trouble.
My little brother was playing CounterStrike 1.6 a lot lately and the game started to stutter very badly. He still had 100 fps solid (fps_max 100), so it wasn't a performance issue. Turns out the video memory is no longer able to run at 560 (x800xtpe). I downclocked it to 540 and everything is fine. But fixing that single issue required a big look into system stability.
Often, the symptoms will give a clue as to the problem. If you could describe the "unplayability" in more depth, we would be better suited to assist you. I'm not sure what "suns lagging maybe" means...