Has any prior accident been caused by a psychologically unstable and/or suicidal crew member?
Confirmed (or Highly Probable)


LAM Mozambique Airlines Flight 470: A report from the government of Mozambique states that the captain deliberately crashed an Embraer 190 in Namibia on November 29, 2013. Cockpit voice recorders were the primary evidence in the report, which found that the other crew members were locked out of the cockpit by the captain. There were 33 fatalities, and Mozambique Airlines was banned from flying in EU airspace. No psychological overview was done of the officer.
EgyptAir Flight 990: A Boeing 767 crashed near Nantucket (Massachusetts) on October 31, 1999, killing 217 people. It was found that the first officer, was responsible. The NTSB and the Egyptian Civil Aviation Authority both conducted individual investigations. The Egyptian report states


  [the] First Officer . . . passed his full medical examination and psychiatric
  assessment, which was performed by the Egyptian Air Force Medical Council as part of his full medical examination for fitness to be a pilot in the Egyptian Air Force, on December 11, 1958. He passed his medical examination for his Commercial Pilot license on July 27, 1961. A psychiatric assessment was not conducted as part of the medical examination for the relief first officer’s medical review for his Commercial Pilot license. During his military and civilian flying career there was no reported history of psychiatric consultation nor any reports regarding his behavior, either professionally or in groups.
  . . .
  First, there is no evidence to support a conclusion that the First Officer intentionally dove the airplane into 142 the ocean in fact, the evidence available refutes such a theory, a determination confirmed by expert medical opinion, technical and human performance analysis.


The NTSB did not agree that the most probable cause was a mechanical failure. Their conclusions, and the relevant rebuttals by the Egyptian investigators, can also be found here.
SilkAir Flight 185: On December 19, 1997, a 737 operated by SilkAir crashed in Indonesia, killing all 104 people on board. The aircraft was almost entirely obliterated. The Indonesian NTSC report (link broken at the moment) said that the cause was impossible to determine. The pilot appeared to be mentally stable, but there have been some doubts. Cockpit recorders stopped at some time, making the case different from that of, say, JetBlue Airways Flight 191 (see below). The NTSB was not happy, severely criticizing the Indonesian findings. Their analysis seem to rule out mechanical issues.
Royal Air Marco Flight 630: 44 people were killed in a crash in Morocco on August 24, 1994. A government investigation ruled it an intentional suicide
Japan Airlines Flight 350: The crash of a DC-8 on February, 1982, was caused by actions taken by the captain before being restrained by other crew members. Because of their actions, the majority of the people on board survived.
1999 Air Botswana Incident: On October 11, 1999, an Air Botswana pilot deliberately crashed an Aérospatiale ATR-42 into two other planes on the tarmac at Sir Seretse Khama airport, killing only the pilot. He was the only one on board, having taken the plane rogue and then flown it about the airport for hours. All the planes were destroyed, though the other two were unoccupied, as the airport had been effectively shut down. The pilot was on medical suspension.


'Close Calls' but no resultant accidents


JetBlue Airways Flight 191: Midway through the flight, the captain of the aircraft appeared to suffer a nervous breakdown. He began yelling about religion and was eventually subdued by passengers after being locked out of the cockpit by the first officer. He was charged with "interference with a flight crew", according to the FBI, which said


  As the plane was leaving JFK and climbing in altitude in its scheduled five-hour flight, [the captain] said something to the first officer (FO) about being evaluated by someone, but the FO did not know what he meant. [The captain] then talked about his church and the need to “focus” and asked the FO to take the controls and work the radios. [The captain] began talking about religion, but, according to the FO, his statements were not coherent.



I also found the same incidences as cpast, from Yahoo! News.



On some of the flights listed, the results of the investigations deemed the pilots obviously mentally unstable. On others, though, the conclusions were disputed, and it may not be possible to truly prove the cases one way or another.