Indoor Trainer Low HR

I have been using an indoor trainer for about a month now, and have been doing HR based workouts after finding my LT. A typical workout is a 15 minute warm-up, some interval work at 170 bpm, spinning for about 15 mins at 155-160 BPM, then finish with 5 mins at 165-170 followed by a cool down. Everything has been going well with the workouts, however, the last two workouts, I just cannot get my HR up above 160. My power numbers are slowly rising (average power has risen about 8 watts) but when i get into an interval my heart rate just wont rise. I increase the power with the hopes of increasing my HR which works a little, but eventually RMP drops below 80, so I decrease the power. Now to get to my questions:

1) Why is there such a drop in my heartrate? I used to have no problem getting it up to 170 or higher, and my outdoor rides I can get it that high still.
2) Should I drop the power once RPM drops below 80, or should I let RPM keep dropping? How far should I let RPM drop?
3) The limiting factor seems to be leg strength, however, my average power output is rising, is it normal that when aerobic fitness improves, strength becomes the limiting element?

Thanks in advance for all the responses, I really just want to make sure that I am not going to screw anything up if I keep training like this.