What are episodic mobility problems as Queen misses State Opening of Parliament

ok.co.uk:

Tuesday 10 May marked the State Opening of Parliament and only the third time since her coronation that the Queen wasn’t in attendance.

Previously Her Majesty, 96, missed State Openings in 1959 when she was pregnant with Prince Andrew, and in 1963 when she was pregnant with Prince Edward.This year, however, the Queen was represented by Prince Charles, with Prince William also in attendance.

On Monday 9 May, a Buckingham Palace spokesperson said: “The Queen continues to experience episodic mobility problems, and in consultation with her doctors has reluctantly decided that she will not attend the State Opening of Parliament tomorrow. "At Her Majesty’s request, and with the agreement of the relevant authorities, The Prince of Wales will read The Queen’s speech on Her Majesty’s behalf, with The Duke of Cambridge also in attendance." At 96, the Queen is very much past the age of retirement in the UK so it comes as no surprise when health issues are cited as reasons for her not attending events.

The word ‘episodic’ means “symptom-free periods that alternate with the presence of symptoms”. Thus the statement from Buckingham Palace means that the Queen occasionally struggles with mobility issues, but doesn't always have problems getting around.