RPMs drop when car is stopped and I turn the wheel left or right (2000 Focus)

I can re-produce this problem every time. Has anyone seen this, can it be fixed as a 'do-it-yourself' kinda project?
I have a year 2000 Ford Focus SE. If I put the car in park and turn
the wheel just a little to the left or right, the RPMs drop and it feels like the car is going to stall but it doesn't. This started happening about 2 months ago. Is there a sensor that knows when the power steering is in use and maybe the sensor is damaged and not telling the engine to compensate by raising the RPMs?! I've also noticed it by driving down the highway and going from 65MPH to a dead stop, all while maintaining a straight line and as my car comes to a stop, the RPMs drop again. The >latest< recall replaced the fuel filter so I guess that can be ruled out as guilty. :)
Any ideas?
On another note.... when should I change my factory spark plugs? (car currently has 55K miles)....
And something else.... I've noticed that on my battery, one of the terminals has that white chalky powder building up around it. Is this a sign the battery need replaced? I had this problem on a '94 Probe and had to replace the battery AND the wires. The mechanic explained it to me but it was some weird answer. Something about the wires building up resistance (or something like that) and even with just a new battery, it would happen again he said.
Oh well, any help appreciated!!

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