Naturopath told me to drink any of the teas designed for sleep to help slow heart rate. They usually include valerian, hops or camomile. You can get valerian tablets as well, designed to be taken at bedtime as a natural sleep aid bit I have taken them in the daytime too. Mot sure you could drive on them though so factor that in.
Magnesium glycinate or citrate may also help. Be careful what type of magnesium you use as some are more stimulating than other, eg magnesium malate is best taken in the morning.
Alternatively, I've just started using a beta blocker called propanalol on an hoc basis when this happens. Have to get it from the doctor but it is effective. Tablet effects supposedly start to wear off after four hours but mine seems to last the whole day so I've found worth it.