Just for the record....
I finally took the camera to Canon Singapore today. As Murphy would predict, when I tried to show them the problem, it took several minutes and 20 shots before it would start to act up. That's life. Eventually, the guy at the counter took it to a technician who pronounced it as "working normally".
I explained that it didn't work that way when I got it, it is a recent phenomonon. Also, it usually starts going wrong about the 5th shot within a few minutes when the with flash is on. In never goes bad as soon as I turn on a cold camera or if the flash is turned off.
Unfortunately, he swore that "it just takes longer to write certain picture files to the memory card". He said that several other people have thought it was a problem with their Powershot too but even when they replaced or "repaired" the cameras under warranty, the new/fixed camera will have the same "function".
He says it normal for that series of cameras. I only agreed that it may happen to all of them but it's not normal - it's just an defect that they can't or won't fix.
I'll keep using the A540 underwater for now because of the housing and also I don't often need to take 2 shots within 10 seconds of each other but I'll definitely stop reccomending the Canon Powershot series to students or forumers. This is a bad defect that Canon says is typical of the range.