Again, "adding a few more feet of vent pipe" will do absolutely
nothing.

Sudsing at the base of a stack "is" a common problem - check this
newsgroup.

My 4" drain has perfect pitch, is not colapsed, etc. What happens is
that the soap/water hits the base of the stack (right where the tiolet
is.) There are enough suds (not a "little" as you claim) to
effectively stop proper venting.

One way to fix this could possibly be to have the washing machine
discharge further down the line or move the tiolet further away
perpendicular from the main waste line or slow the discharge rate of
the washing machine.

The house was built in 1968 and was all done to code period. I've
lived there for over 10 years and have never had a problem. I don't
think that Tide with fabric softner existed in 1968 and being that it
is the only type that causes this problem I don't necessarily fault the
plumbing.