Whilst it is difficult to be certain what is going on without recording an ECG at the time of your apparent slow heartbeat, almost certainly what is happening is that you are getting ventricular ectopic beats. These are “premature” beats which do not produce heart contraction strong enough to be felt at your wrist when you take your pulse. If they are repetitively after every paced beat, a pattern known as bigeminy, then the pulse counted at the wrist appears to be half what it actually is. This pattern can cause symptoms such as tiredness. Because your pacemaker detects that your heart is beating on its own (and ventricular ectopic beats can happen despite heart block) then your pulse rate can appear to be below the programmed rate of 70.
There are other possibilities related to pacemaker programming and function, so that the best way to be sure what is going on is for you to be attached to a pacemaker programmer while you have are having symptoms. If this happens only occasionally then a 24 hour Holter or one week event monitor might be needed. I am very surprised by the number of normal angiograms you've had!
Depending on what is going on treatment may range from nothing to changing your pacemaker program. Medication is really required or necessary.
By the way, there is no such thing as a “pre-emptive” beat. Maybe you misheard “premature” as this. I'd been talking to your cardiac electric physiologist/pacemaker doctor rather than the technician.