Mystery instability of 16f628a

[Davidmarks]

I do not believe this ! After a spot of tinkering and adding to the circuit this am it has suddenly gone unstable again (It only operates when finger approaches or touches PIC) stripped circuit down to basic (schematic attached) and it still doesn't work. By the time i have finished it would be quicker to build the thing with valves (tubes) I used to understand them ! The code is as follows:
[@ device pic16f628a,INTRC_OSC_NOCLKOUT
define osc 4
begin:
low porta.0 ' Turn LED on
PAUSE 500 ' Wait 500mS (1/2 second)
high porta.0 ' Turn LED off
PAUSE 500 ' Wait 500mS (1/2 second)
GOTO BEGIN ' Loop continuously
end