Ive had my 705 for 1 year now, and here is what Ive found:
1. HR spikes - I wasnt wearing the strap tight enough, plus wasnt moistening the contacts - now I just use warm tap water and smear over the contacts prior to putting it on - no more spikes.
2. Cadence spikes - I found the cadence sensor was very picky with how close the crank arm magnet was - readjusted and now everything is sweet.
3. Speed spikes - similarly I found the wheel magnet needs to be very very close to the pickup. My issue is that with my wheels, even with the Dura Ace slim line magnet, I couldnt align the magnet directly opposite the pickup arm. I have since resolved this my having the magnet just clear the END of the pickup arm on the sensor.
I have turned off the auto-lap sensor, as I ride many different routes, most rolling hills, so comparing each 5km segment as a lap doesnt really make sense for me.
I have auto-pause turned on all the time, and only turn on the auto-lap/start if Im doing interval lap courses.
For lap/split times I load my Garmin data into SportTracks, and via the AfterImport plugin, I can define auto-split points on a map.
I keep a file copy of every history file on my computer for archive purposes, load the history into Garmin Connect, and import into Sportracks, then delete the history file off the Garmin, to keep it clean and fast.
I also keep a library of courses on the computer, and only load a subset of courses onto the Garmin at any one point in time.
Occasionally I have the 705 screen suddenly go blank on me during a ride. I found if I power off the unit, then power back on, it seems to retain my current course or recorded history. The only trick is to remember to push the start button, as it seems after the reboot it wont kick out of auto-pause.
Oh, and NEVER automatically update the firmware on your Garmin unit. Always wait for 2 weeks or more, check the Garmin forum to see how many people scream about issues - if hardly any faults are raised, then its POSSIBLY safe to update