Garmin recording ludicrously high speeds?

My 405 has recently started recording silly high speeds, which is preventing me from being able to tell what my real highest speed is on training runs. Looking at my mile splits from each run, it always happens in the first mile, which makes me think it's probably happening right at the start. All the rest of the functions are working fine and all the rest of the data it records seem to be accurate.

I thought it might be caused by starting the timer before it had locked in on the satellite signal, thus confusing it as to how fast I was actually going, but yesterday I made sure it was locked in and ready to go well before I started running, and it still happened.

My 'top speed' yesterday was 17mph, last week it was 243mph. Does anyone have any idea what might be going on and how to get it sorted out?

Can't help you, I use a 110.

Tonight it recorded 4,024.4.mph. WTF???

Well, when I start it up it spends a couple of minutes finding a signal. A little bar on the screen shows this happening, then when it's found the signals it needs it just goes straight to the time/pace/distance screen. So I assumed it was properly homed in before starting, and looking at my route maps it is recording my routes right from the start.

I emailed Garmin support about getting a new battery put in about 6 months ago. (I don't think that's the problem here, I just wanted a better battery life on long runs.) I knew it was out of warranty so said I'd be happy to pay for it. Their reply just said 'Sorry, your unit is out of warranty.' I emailed them back again saying yes, I know, but I'm offering to pay for the service if you can provide it. And they replied again saying the exact same thing: 'Sorry, your unit is out of warranty.' I wasn't very chuffed with their lack of helpfulness...

But I shall try again! Thanks for the suggestions...

Silly question, but have you tried resetting it? Pressing both buttons on the side at once until the screen goes blank?