If you had a corroded wire it may have caused the starter to draw excessive current, and now it may be intermittently failing. The correct procedure to diagnose this condition would require the problem be evident while taking voltage measurements at the starter. This would ellimnate the wiring, starter relay, battery, ignition switch, neutral safety switch, and the anti-theft which all play a role in starter operation. If you here a "clicking" under the hood this may be an indication the starter isn't failing. Also a intermitently failing starter will fail more likely if you continuously turn the key to crank than shut the engine off, repeat at least 10 times in a row, the starter may at one time not engage and if not, you can take it to the shop and show them you can get it to fail so they can make there checks. Also, check the connections at the starter soilinoid for corrosion and loosness. The next occurance, you may want to attempt starting it in nuetral, if it does the neutral safety switch may be at fault, or push the shifter further into park and attempt cranking.

-Rip