I was exactly the same, fast resting pulse and waking up in the night with heart racing and in my case sweats, thankfully this didn't happen every night. I reckon light exercise in the form of walking and the deep breathing exercise @underwater mentions helped me since 4 3/4 months in it's gone. I also cut down caffeine and reduced my sugar intake too. The high pulse seemed to be part of the anxiety symptom (the signature of PCS!). It's like I had to retrain my body to increase and decrease my pulse according to physical demand because it had got stuck lol.