How much socialization have they had? Insecure dogs and dogs without a lot of socialization can easily become fearful in new situations with strange people and dogs. The fearful dog has to, therefore, choose between flight and fight. When the dog is on a lead, however, its only option is fight. Because they have an aversion to dogs, their main goal is to try to get away from it. When that option is out, they can only try to drive the other dog away by growling and attacking. Every time this behavior happens and every time it is successful, the fear and bad behavior is reinforced. I have a feeling that this is their reason for on/off good behavior. Ivy was going through the same thing, except she actually physically attacked dogs.

To combat her fear, we went through heavy socialization. I mean we exposed her to EVERYTHING. In addition to that, we frequented large, off-leash dog parks but kept her on-lead. For the first few times, we had to pick and choose which dogs she could associate with (calm, friendly, submissive types). I would keep my grip loose and calm and I'd let her meet the dog. After she was done sniffing, I pulled her back towards me and treated her before she could attack. Every dog has its limits, and Ivy's was that she could not spend an extended amount of time being friendly on-lead. So I cut the exposure short to prevent any attacks and thus prevent any reinforcement of bad behavior.

Also, you might want to try this with each dog separately. Dogs in packs are generally more unpredictable. Then, again, your dogs may not be fearful and this may not be the cause, but I hope it gives you a lead!