Three months ago, Kalvin A. Stafford agreed to plead guilty to killing his 19-year-old girlfriend last October in Montgomery County.

But yesterday, Stafford changed his mind. He fired his attorney and asked Circuit Court Judge William C. Miller to nullify the plea agreement.

The reason, he claims, is recent "evidence" that his girlfriend may still be alive.

Stafford, who filed the court documents on his own, said his former attorney "should not have convinced" him to accept the plea agreement because Judge Miller had received a letter allegedly from the victim, whose body was never found. In a two-page motion, Stafford also said Miller "is alleged to have received a telephone call from the alleged murder victim."

Stafford's actions have upset the victim's family, and could set the stage for the second "no body" murder trial in Montgomery County in two years.

In a statement of charges filed by Montgomery police, Stafford told authorities he choked Anita F. Bowling to death on Oct. 2 during an early morning argument over money at a Silver Spring apartment they shared.

Stafford, 33, a computer programmer for the federal government, told police he disposed of Bowling's body in a dumpster in Columbia, about 20 miles away. Police believe the body was later burned in an incincerator in Baltimore.

Law enforcement sources acknowledge the receipt of an unsigned, typewritten letter purported to be from the murder victim.

The letter, received several weeks after Stafford's April 4 plea bargain arrangement, was postmarked from Connecticut, a source said.

According to an official involved in the investigation, the letter "essentially says, I'm alive, free Kalvin Stafford."

The law enforcement official said police have discounted the letter as a "nice try" by someone trying to set up an alibi for Stafford. A source close to Miller denied the judge received a phone call from anyone claiming to be Bowling.

Edward G. Varrone, the attorney dismissed by Stafford, declined yesterday to discuss the letter. In a previous conversation, Varrone put little credence in the document. "I'm not spending a lot of time on this," he said.

Miller granted Stafford's request yesterday to remove Varrone from the case and agreed to appoint a public defender to represent the former Army sergeant. Miller set an Aug. 29 hearing on Stafford's motion to withdraw the plea agreement.

If the Stafford case goes to trial, it would be the third "no body" case since 1983 in Montgomery.

Last November, a county jury convicted Gregory Tu of first-degree murder in the 1988 killing of his missing common-law wife based largely on circumstantial evidence. Tu was sentenced in January to life in prison.

In the county's only other no-body murder case, William Hurley was charged in 1983 with killing his estranged wife.

He was later convicted of manslaughter, although the woman's remains were found in 1986. Hurley was sentenced to 10 years in prison.