I've had very good luck with this strap:
https://www.amazon.com/gp/product/B00BI9X1QM/ - comes with the xmtr unfortunately or perhaps fortunately.
and this strap:
https://www.heartratemonitorsusa.com...nt=11309945414

I always use gel on the bike. The strap has to be tight enough. If you've lost weight, maybe tighten it? If HR is dropping out, might be a low battery. Replace it?

I've been using HRMs for 20 years, never had a problem with high HR dropping out, so something unusual is happening. The above Garmin strap is particularly nice - never had a problem with jersey flap with that one. Polar and Garmin straps using snap-on transmitters are interchangeable, though not the transmitters.