This is a charactaristic of LCD displays, it's not a fault with the Omnigo.
The one time this happened to me (I put it down in the passenger footwell
of my car such that it was resting against the gearbox (which gets rather
warm)) I was able to compensate by adjusting the contrast (hold down ON and
press the + or - keys). Other than not letting the Omnigo get hot there's
not a lot you can do about it.
Asher.