Welcome, Yonghoon! Congratulations on getting your Spoiler, but we are sorry to hear you are having trouble with it.
I'm not really the most expert on these things (they should be here soon), but I will offer a few suggestions.
Generally, it sounds like you have some sort of connection problem. At first, it was always bad; after the initial repair, it worked sometimes but still has bong~bong~ days. So now it is an intermittent problem, which suggests that maybe just moving things around helped a little, but did not really address the problem.
With all respect to your Engineer, I doubt that the magnets in the pickups wore out. Sometimes a winding (wire in the coil) may fail, or more likely (but still not very often) the wire leads coming out of the pickup may become disconnected from the coil.
If you are lucky enough to still have the original pickups, someone here can tell you how to easily test them. It may turn out that the pickups were actually okay, in which case you might be able to sell them to someone here (or at least keep them as spares).
You say that the bad volume pot and knob were replaced. Do you mean just the volume pot, and the knob that goes with it, or was some other control replaced besides the volume? Sometimes the pots get dirty, and since they are sealed you cannot clean them. The recommended procedure is to rotate them back and forth, 20 or 30 times, to clean them up, and this often seems to work. You should do this with the filter frequency pot (I think that's the only other pot you have on a spoiler).
What about the pickup selector knob? Since the original problem seemed different depending on the pickup selection, I would suspect that dirty or worn contacts in this switch could be a problem.
In general, you should clean any electrical contacts you can get to, including any modular connector plugs and the instrument cable jack, and exercise any knobs or toggles that you cannot get inside to clean.
You might also experiment a little more. On the days when it sounds bad, does it suddenly get better if you switch pickups, toggle the Q switch, or change the volume? Or is it really just random?
And of course, make sure you have a full strength battery, and good connection there. I'm probably forgetting something, but others will join in with suggestions for you.
You may need to replace the circuit board, but make sure you've considered everything else before doing so.
Good luck, and let us know how it goes.
-Bob
