The area to look at like 360Alaska stated is those parts. But also there are 2 cable jacks to look at, most importantly the one that connects to the CRT Yoke. It should be 6 to 8 pins and the solder on a couple of those pins crack over time. You need to desolder, clean up the area and resolder with fresh solder. This will give a strong repair that will last a few years. Reflowing the solder will temporarily fix it for a couple of week to a month - depending on how often and how long you have Mac on.
While you are there, look at the top of the analog board where T1 is at and desolder, clean and resolder any suspect joints there too. These two areas are connected and affect each other. If you have cracked solder joints at one place, you will have solder joints going bad at the other.
You do not need a hot air station for this, just a simple iron, solder wick, flux, and a roll of solder. It should only take an hour at most to do. 1/2 hour if you're with an iron.
