@Albert-Prat Without knowing anything about the watch you use, the environment or how OHR usually works for you: The first problem sounds like ‘cadense lock’ where the sensor mistakenly interprets the arm swing (or body vibration) from your steps as the actual heart rate. In warmer weather it usually can be corrected by tightening the watch strap and making sure it sits a bit above the bony protrusion bewteen wrist and hand. In cold weather, due to the fact that the blood vessels contract and are pulled deeper below the skin, this sensor confusion is more likely to happen. Even I, with very little melanin in the skin and usually having good results with OHR, use an electrical signals heart rate strap between October and May (living in Sweden).
A heart rate strap having spikes at the beginning of a run could be due to dryness; not having a good contact with the skin. Also more frequent during winter with dry air. I always run the whole strap under the water faucet/tap before putting it on. During the exercise the body sweat usually suffice to keep a good contact between electrodes and skin.
Poor HR strap performance can also be caused by low battery levels, contaminated electrodes (dirt/salt/minerals) or them just having lost conductivity due to wear. I bought a new strap (the pod is fine) for my Polar H10 this season, the fourth year after original purchase. The electrodes had begun to crack and lift a bit - which I first glued down again - and I simply lost faith in the overall integrity.