Wobbly boxer

My 3 year old male boxer has several issues. A couple weeks ago he was lethargic and his gums were almost white so we took him to our the vet where blood work (including tick born disease test) were done. His red cells were low but a second blood work test showed they were coming back to normal and he did NOT have the tick born disease. A couple days after this issue, he was playing (jumping up) and suddently stopped and just fell to his side and they did an EKG and his heart looked fine, but the vet recommended an ultrasound at a specialist vet clinic, which we have scheduled for tomorrow (Thursday). BUT, I also wonder if it could be something else? I have witnessed a number of times where he gets up from sleeping/resting (which he does a lot!) and then stops suddenly and his legs are wobbly. He just stands still for about 30-45 seconds and then he seems okay. His pees are also very long....very long! We lost a boxer to cancer so I'm familiar with lots of dog issues, but not this one. Before I spend $500 on the cardio ultrasound, are there other things I should be checking first? Thanks so much for taking my question. BTW, Beau is brindle and weighs 82 lbs. He is eating fine and drinking his water. We feed him good food and a fish oil vitamin twice a day.

At the cardiologist appt today, they said on a scale of 1-6 he does have a heart murmur (level 3), but the ultrasound of the heart was okay. They see a very minor congenital defect (subaortic stenosis) when the blood flow leaving the left side of the heart out of the aorta is too fast and becomes turbulent. This is a common boxer defect and very mild and the vet does not think this is causing his weakness. We have him on a 24 hour Holter monitor to evaluate his heart rate and rhythm for a day to see if we can find evidence of any arrhythmia that could be treated with meds..but that's it for now. He's not too happy with the Holter monitor..it must be as hard to wear this as the cone of shame! Didn't do the abdomen ultrasound yet - we'll see how the holter reading goes and who knows...but thanks for the info!