It is normal for Ph to drop during the course of a week inbetween water changes. Is it possibile that this is what you are noticing? I can't think of a reason why there would be sudden drop in Ph that would be caused by picking a fish up off the floor and putting it back into the tank. Did you test the water just before it jumped and immediatly after you put it back in?
Agree with Tom, and if you have plants (although with an Oscar I doubt it) it is normal for pH to be higher in the evening and lower in the morning by a few 10ths.
You also may want to increase the frequency of water changes, and gravel vacuuming, if pH is drastically different from one water change to another, the acidification could be putting stress on your fish.