Hi joethemechanic,
is sure is irritating to watch seemingly random acts triggered out of nowhere. There is another side to impulses however: I suddenly felt an impulse to respond to your post - maybe impulses are a good thing?
Accepting that our SO is driven a lot more by impulses and different from us is an important first step. While it drives us mad to see impulsive boundary crossing the same impulsiveness may have led to us connecting to each other. Impulsiveness is a fairly fundamental part of how a person operates and won't change much. (That is not saying people can't learn
how they act on impulses). And we may even love our partner for being spontaneous.

Controlling impulses is virtually impossible, trying to do so is controlling (invalidating leading to dysregulation), channeling them into something useful is not likely to happen in the short term. So... .
We can aim at limiting damage on our side from impulses - boundaries are critical. That is one reason boundaries need to be under our control otherwise they are "spontaneously" crossed.
We can aim at a calmer validating environment and hope it is less eliciting impulses and providing more time and space for evaluating the impact of acting upon the impulse.
Ultimately a pwBPD needs to adopt own an own set of boundaries aka. growing up - yes this is a great car and I want to buy it - no, won't act on it today as it is way over my budget and the current car is only a year old. This may take therapy and definitely some time.
I think this is our biggest source of conflict. It's like she runs her life as a series of impulses. How she has kept the same job for 15 years is beyond me. And she really doesn't understand why I get angry about her continual changing of plans.
Knowing she is not reliable - can you plan in a way that lets the consequences of changes to plans fall on her side?