The son of tragic former DIFF'RENT STROKES star DANA PLATO is suing the man his mother was with when she died from a drug overdose in 1999.

TYLER LAMBERT, 22, has filed a wrongful death suit against ROBERT MENCHACA, claiming he's responsible for the actress' death - because he failed to act quickly enough to save her.

Lambert says, "I don't believe she killed herself."

Plato's angry son is urging police to re-open the case into his mother's death, believing there was a cover up in Oklahoma, where she died, according to US tabloid National Enquirer.

Lambert has asked for a change of venue to Clark County, Nevada because he believes he'll get a fairer trial there.

In his suit, Lambert is seeking general and punitive damages, claiming he has been deprived of "companionship, affection, love and aid in maintaining a normal life."

Plato played rich girl KIMBERLY DRUMMOND in the hit 1980s sitcom.