huge error of cross section with calchep (w+w- collision)

The main problem, you try to calculate cross section of collision of unstable particles. Look at last amplitude diagram.
W+ could decay o ne,E and after that on-shall E interacts with W-. There is a paper by Serbo and Melnikov which find a way to treat this problem for the case of muon collider. They take into account finite size of muon beam and processes where decay
takes a place far from point of muon collision does not contribute to total cross section because products of decay leave the beam.

Second problem is infinite cross section of elastic scattering of charged particles. You actually have a subprocess
 W+,W- -> W+,W- whose cross section is infinite. You can solve this problem applying cut for transverse momentum of {e,Ne }

Other problem which appears here is A->e,E subprocess which also diverges if Me=0. But the last problem can be treated via
electron mass or implementation a cut for total mass of e,E couple.