Change Timex Ironman Watch Battery

Runner needs help with a Timex heart rate monitor?

I have a 1 year old Timex Ironman heart rate monitor-digital watch combo with chest strap. For the past month or so, my heart rate monitor reading jumps to 240 beat per minute without warning and stays there for the majority of my run. My usual avg. heart rate ranges from 150-180 bpm. I tried changing both batteries but it still does not solve the issue. Can anyone advise if they had this problem, and if so, what was done to fix it? Thanks…

I have a lot of hair. I wet my chest with salt water.

Timex Sport Collection Ironman Resin 19mm Band