weird and funny but not sure what!

[scorpio]

the external temp reading on the dash was fine when I started out, then it dropped to -9.5 as I carried on it started to rise slowly.

Parked up and on return the reading was normal, about 20 miles into the journey it dropped to -7 then slowly started to rise again.

This has happened three times now but every time i start the car it shows the real temp outside, does the sender fail slowly or do i need to look elsewhere.