When sitting still, apple watch heart rate jumping from 90bpm to 39 to 80 to 45 etc within seconds. only stays at low bpm for 5 seconds or so. usually stays around 70-80bpm but then will start to have quick dips before stabilizing again. concerning?

You're better off learning to take your pulse as you can't hang your hat on those rapidly fluctuating pulse rates. You'd need to wear an outpatient cardiac monitor for 24-48 hours to get accurate heart rates and rule out arrhythmias when your watch is demonstrating widely varying pulse rates. I think your watch isn't sensing accurately at times.