Big Variation while riding

Dear all,

A friend of mine just gave me a pair of V3, I tried them this morning.

I noticed a big variation of +/- 30 watts while I'm putting the same power and going at a constent pace (on flat, and little paced climb of 2km).

For example: trying to ride at a constant "feeling" of 220 watts, my Edge can tell me 180 watts, and immediately after, 250watts

Same while climbing at a "feeling" of 300watts, it tells me 280watts and the second after 340 watts.

Any idea how I can fix that ?

Thanks everybody.