Wireless Odometer

I have a wireless odometer for my folding bike, and I notice that when
I start riding it works fine, but that when I stop for an extended
period of time it seems as if it malfunctions. It will say I am
traveling at 10mph, and then a second later say zero. I thought maybe
the heat is affecting it, since we have been having high 80's with
high humidity. I took the handle bar part off while I ate lunch,
figuring it might heat up outside, but that did not seem to make a
difference. I have a wired on for my other bike, with no problem. I
would have used a wired one for my folding bike, but I figured that
with folding it and putting the bike in the car it would not be long
before I broke the wires. I check the sensor on the fork, and it seems
to stay in position. Any suggestions? I usually ride my folding bike
on crushed limestone, but it does the same on asphalt. It is an low
priced model.,

Thanks

Tom