I have the same problem, actually our numbers are almost identical.
When i have an 'episode' it generally lasts for an hour or so and even when my bp gets back to a respectable level i am left feeling drained and like c*ap for at least the rest of the day
I was the same with my last pregnancy, and my ob pretty much said theres nothing you can do, its the better problem to have when it comes to bp. just be mindful that it occurs, get to know the feeling when its coming on, so you can put yourself in a safe place in case you do faint, to follow your body and what it wants, so if you feel you need to stop and sit down do it, if you feel you cant possibly get back up then dont.
The things she told me which i found did help keep my bp at a decent level and reduce the amount of times it dipped were water intake to be at least 2ltrs a day, more if possible and increase salt intake, which for me isnt hard as i dont use salt to cook my food, or have it on it, so when im pregnant i generally cook my vegies with salt in the water and season my food more, however this time around i havent added salt to anything because of DD, i have found my bp dipping many more times throughout this pregnancy, in fact it had stopped altogether by the third trimester last time, whereas this time it's happening at least once a week sometimes 2-3 times a week.
I would mention it to your ob next visit and see what they say, each person/pregnancy/ob is different.