Regulators that perform well at depths below 70 feet

I have recently come to the conclusion, after careful observation, that my
regulator does not handle breathing at depths beyond 70 feet very well. The
effort required to pull air gets rather hard. When I dive above 70' I can
get a total dive time of over 40 minutes. But as soon as I go below that
level my total times falls below 30 minutes, and it does not seem to matter
how long I am below that depth, even just a couple of minutes will do this.
Wondering what might be behind this I tried to compare the effort in
breathing through my regulator at 60' and 80' and I found a noticable
difference. The draw was stiff, I had to put extra work into breathing that
then caused me to increase my breathing rate, kind of like I was running,
yet I was not physically working to fight current or such. In fact I was
cruising with the current or in relative calm and not working.

I think it might be time to explore replacing my regulator (which is a basic
regulator purchased in 1997) with one that is more capable of handleing
depths below 70'. I'd like to hear from other divers what regulator they
use and their experience with it below 70'.

Thank you for any responses.

Walter Willis