I start a singleplayer offline server on the bedroom map, when I look in certain directions my game suddenly gets very choppy. Whats worse is it sometimes changes positions, so I can look in the direction that was making it choppy and it is fine. Though when I look in another direction the same thing starts happening. In red_guys unlimited mining I rarely lag, the only lag I would experience is if falling down a tunnel at the highest velocity possible while having to ghost all the bricks as I fell. - (That was before this odd thing started happening)
This problem has effected me online too. Same thing, looking in a certain direction causes my fps to drop and me to lag, when I look away from that direction i'm fine. Then it changes areas and it's just frustrating.

All my drivers are updated.
Removed ALL add-ons; to no avail.
Reinstalled; to no avail.
Turned all graphic settings to lowest possible; to no avail.
Searched for possible solutions; to no avail.
Specs - Windows vista home premium 32 bit; AMD Athlon(tm) Processor LE-1640 2.70 ghz
4.00 GB ram, ATI Radeon HD 4650
This problem started a month ago (estimate) and has been bothering me ever since. None of my other games have had any performance differences.