My history with SRM's is:
1. Wired Cannondale Si with PC-V, which worked flawlessly, all the time. I bought it second hand and later sold it to a friend.
2. Wireless Cannondale SiSl with PC-VII, which had dropout issues which I thought was due to my positioning of the magnet. I bought this new, but sold it to a friend earlier this year.
3. Wireless Rotor 3D+ with PC-VII, which I bought last year from the original owner. It won't connect to any of my Garmins - 800, 910XT, Fenix 2, so all figures are from my PC-VII.
I only spot this happening when I am accelerating up a hill and look down to check what power I am averaging- i.e. can I maintain this, should I push etc.
The power reading changes to zero, then "-", where it stays for a while, then it changes to zero again and finally back to (say) 326.
It happened today, but I didn't think to look to see if the cadence dropped out at the same time, but looking at the data in Training Peaks I can see that both power and cadence dropped to zero at the same time.
Looking at this, notice where speed increases up hill, with zero power and cadence:

This is, not to put too fine a point on it, driving me nuts.
Is it likely that this is a magnet placement issue, a reed switch issue, both or neither?
Also, why won't the SRM speak to any Garmin devices? My Cannondale ANT+ SRM did.
I've also got a P2M and a Powertap, and they a) both work flawlessly with no dropouts and b) speak to any Garmin.