  We consider the two-dimensional quantum field theory of a scalar field
self-interacting via two periodic terms of frequencies $\alpha$ and $\beta$.
Looking at the theory as a perturbed Sine-Gordon model, we use Form Factor
Perturbation Theory to analyse the evolution of the spectrum of particle
excitations. We show how, within this formalism, the non-locality of the
perturbation with respect to the solitons is responsible for their confinement
in the perturbed theory. The effects of the frequency ratio $\alpha/\beta$
being a rational or irrational number and the occurrence of massless flows from
the gaussian to the Ising fixed point are also discussed. A generalisation of
the Ashkin-Teller model and the massive Schwinger model are presented as
examples of application of the formalism.
