Are you saying while working at a constant/level exercise aerobic level your HR will drop from 180 to 34? This sounds like equipment failure to me.
I did read that you get these strange readings on different equipment, but are you using the same transmitter? For example, my Polar wrist monitor with a chest strap will also drive the HR on some tread mills and other equipment, so if the chest strap/transmitter is producing errors, those errors will be displayed on all compatible equipment.
Yes, while working at a constant level my heart rate drops. Most exercise heart rate monitors go blank as the heart rate goes below 60. Then a few minutes later it starts recording at about 110 to 125. Other times it jumps up to 180 and then back down. It has been happening on numerous machines for several years. No chest strap on.
I agree with Jerry. This sounds like equipment issues. Have you ever jumped off the machine when this happens and check your rate with your fingers?
I'm thinking equipment, too. If there's no chest strap, it's either a tethered wrist strap or the hand sensor things, all of which I've found pretty unreliable.
Even with a chest strap I've found occasional odd readings.