You're not alone in encountering this issue, but I would add a couple of caveats.
My 70% whr is 137. I found that doing lots and lots of very slow running led to repetitive strain injuries. It also over time led to hr drift. My suggestion is once a week to do something more energetic, to "reset" the body, so to speak, like rebooting the computer. I found that the 70% run the day after one of these sessions was at a faster pace than before and the repetitive strain niggles went away.
Also, we're now heading into the warmer weather and the body's increased need to cool itself means it works a bit harder and that might mask any gains in the meantime. Put in the mileage in summer and the results will be seen in autumn.
As for keeping the hr steady, I found that near-impossible due to gradients etc. I go by the average hr for a run.