fits of tachycardia and jogging

I am 48 and go jogging in gym regularly for l8 months. I use Polar Heart Monitor and it shows me, that after 20 minutes of slow jog (7-8km-h) I started having asymptomatic tachycardia somewhere between 150-207. During exercise my baseline HR is 106 and it jumps to those high frequencies. I stop jogging and it disappears. After few slow paces it is back again. I have been seen by sport doctor 3 times without any conclusion. When I saw him first time last year,  could run 10km-h, now hardly 7, if it is going on like this, I will be unable to walk without tachycardia.

[mum2four]

My polar HR chest strap some time shift causing it to make high reading when my heart rate is not high I wear a second HR watch(first one I bought) and have double checked using my other watch which is a ECG press butten on no strap. Checking and re-ajusting the strap slightly fixes it.

If it really is your true HR I would not worry too much it may just be you limit for exercise and you've started pushing you body past its current limit. I'd be more worried if you stay above 200 after you stop jogging. Excercise and heart rates can be safe even when high. I think the ratio is 200 beats minus your age. Anything high than that during exercise or after exercise really needs looking in to.