I had read a few places before that static electricity can affect your HRM readings, but hadn't experienced it--I think. Read this post over on the main board. The static electricity is caused by certain types of fabric in the shirts.
Just something to add to the following possible reasons for abnormal readings:
--strap battery getting low
--you mentioned slippage. If it is slipping, that means in isn't on tight enough. This could do it. If you can't get it tight enough, bunch it with a safety pin.
--running with a cel phone on the side of your body where you wear your watch.
--dirty transmitter. Make sure you wash the transmitter and strap once in awhile.
--running under high tension power lines
--not wetting the strap when you start
--having other hrm transmitters too close to you, or certain electrical appliances.
It was also discussed how the dry wnter can give you those spikes in the first mile. It was suggested that
putting the strap on a half hour early can help.
Just passing this stuff on. Most of it has been mentioned in other threads over time in this forum.
It also has been discussed in the past here how for a few people (in the heart rate threads over the years) those spikes were actually real, and they were having heart problems. One a racing heart, and the other ended up with a bypass. SO when it happens, if you can't get it back down, take your pulse to make sure it's erratica and not reality.
Log PRs Crusted Salt comics #192 (NSFW)