Cardas has been recommended in the past, so I will give it another go. Last time, joints didn't look good, but I have found that ALL the previous solder must be removed before you can trust the results of resoldering a joint.
Some of the newer products have solder that is simply hard to work with (high melting point). That is a recent change in the last few years, but it doesn't happen too often. Figured that it was just hi temp no lead solder, but after reading the paper, I see that I was wrong. Glad it was put forth.