Runs in safe mode but spinning beach ball every few seconds in normal mode

hi,
i've been struggling oct 2010 imac running 10.6.8 last few weeks. processor 2.66ghz intel core i5.
4gb 1067 mhz ddr3
cannot upgrade newer os no longer able print 1 of industrial machines. replace machine 25,000 bucks. need keep old machine running long can.
problem began when moved computer 1 office another. new location has number of electrically powered machines high voltage. after few days of struggling took computer home, ran repair disk , worked beautifully @ home. returned office worked 10 minutes , behavior began again.
moved machine out of office location worked beautifully week. yesterday noticed unusual name on shared list in finder window, 1 had never seen before , couldn't figure out was. no unusual activity on wifi network. began having spinning beach ball issue again. maybe coincidence. decide try , hard wire network no difference in operation.
today booted safe mode , computer working beautifully, except of course there things can't because in safe mode can't continue this.
completed "diagnostic" site results gobbelydegoop me.
command
syslog -k sender kernel -k message creq 'gpu d|i/o|nspace-h|n cause: -|timed? ?o' | tail | open -ef
gave me these results.
thu jul 17 12:35:42 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:35:59 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:36:19 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:36:38 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:36:55 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:37:11 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:37:27 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:37:43 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:38:02 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
thu jul 17 12:38:18 ang-thomasqs-imac kernel[0] <debug>: disk0s2: i/o error.
can enlighten me please?

 

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