Why is my golden foaming at the mouth?

Hey all, we have a 2 yr old female golden retriever named Rylee. She is an all around energetic, athletic,and playful golden with almost zero issues up until now. Over the past 3 weeks she has been exhibiting very strange but vague symptoms, hoping for some suggestions and help here.

3wks ago while running up the steps behind me, she stopped midway and I could tell something was wrong. My first instinct was that she was about to pass out, she was on the step almost dizzy. This lasted for only a minute, then she was back to her normal self. I thought maybe she had lost her balance, so I let this one go. This past weekend, while playing tug of war with her favorite rope toy (she loves to wrestle and play kind of rough), she started foaming a bit at the mouth. Not full blown bubbles and foam- but increased drool dripping down on the floor- totally out of character. This alarmed me so I took her to our vet.

The vet suggested it may be the toy, the rubber in the toy giving a bitter or sour taste...but this toy is almost 6 months old and she has never had this problem before. We removed the toy, but just tonight she has had the foaming incident again. When this happens, she stops what she is doing and starts licking her lips. She normally then immediately comes to me and lays on my lap or right against me if we were playing on the floor. I get the feeling she is scared or nervous about what just happened. These episodes seem to always happen while playing, they have not happened any other time. Besides this, she is acting 100% normal. Eating, drinking, and bathroom habits all normal.

Any suggestions or anyone have similar problems? Our vet was very vague- she wasn't sure and said it could be just the rubber toy on one end, to seizures on the other end of the spectrum. I have also read possibly an ear infection making her dizzy while playing hard? Thanks so much for any help/ideas.