So I am going to reflow all of the suspicious Solder joint which look brownish right?

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No you are going to identify the solder joints at the locations I and others specified above. If it is clear that the solder joint was scorched, and not just discolored, then you have a component failure as well. The best way to identify that is to look for scorch marks on the back of the white vinyl panel that covers the analogue board. Otherwise as pointed out earlier, cracks in solder joint are usually not visible without microscopic inspection. If you see any obvious defects, then by all means secure the connections. Otherwise, it is best to proceed location by location. If the places I have relayed from Larry Pina's book to not restore the Classic, I can give you a list of all the trouble spots to investigate. Though from the description of your problem, I gave you only those that applied.