Dina & Michael Lohan's Truce Is OVER -- Explosive Fight in L.A.

So much for new beginnings ... Dina and Michael Lohan's temporary ceasefire came to an explosive end today in L.A. ... after the two went nuclear on each other on the set of a talk show.

Sources tell TMZ, Dina and Michael were taping a segment for "The Test" -- a conflict-resolution talk show launching this fall that uses lie detectors and DNA tests to settle disputes -- when things went South.

We're told things started out great -- but soon the finger pointing began ... and when the topic of infidelity came up, the gloves flew off.

According to sources, both Michael and Dina accused each other of lying about their past ... as well as who's really responsible for Lindsay's downward spiral ... and the whole thing ended in a firestorm. We're told Michael took two lie detector tests, but Dina refused to take any.

Now Dina and Michael -- who seemed to bury the hatchet during a burger date on Wednesday -- are back on terrible terms.

As we reported, Lindsay's treatment at Betty Ford involves family counseling -- and while both parents said they wanted to cooperate ... it looks like it's not gonna happen.