I know this might throw you for a loop but the lack of a ridge vent is not the issue, especially as you have gable vents.
The issue is excess moisture in the attic which requires you to find the source of it and manage it. Primarily you need to make sure your attic is separated from the other levels by proper air sealing and that all your dryer, bath and oven vents actually vent outside and not in the attic.
The next item is managing the moisture you generate, so once everything is vented properly:
- make sure you use those exhaust fans,
- consider lowering the temperature somewhat, and
- if you have a ton of indoor plants or a ventless heater you are going to have reconsider them.
Once you have done all this (and your attic starts drying out), you may also want to look at how much insulation you have and make sure it is installed properly.
For more information:
See my blog post entitled "Ice Dams, Icicles, and Attic Condensation."