Okay guys.
Consulted with a mechanic who has a smoke machine and after describing my problem was told that the engine just was not producing enough vacuum, not that there was an external vacuum leak so using the smoke machine would be a waste of time. He said due to the intermittent /sometimish nature of the "leak", something internal was causing the engine not to produce enough vacuum.
Did some online research and realised that a dirty EGR Valve could be causing the erratic idle and possible vsacuum loss if it failed to close properly.
Using this guide by raybic
viewtopic.php?f=33&t=453517 , i decided to remove the intake mainfold today, clean it using throttle body cleaner, purple blaster and water and finally delete the EGR.
After reassembly, the engine erratic idle has steadied and the vacuum gauge reads a steady 24.
However, once the AC is on, the vacuum gauge starts behaving erratically again.
So back to the drawing board. Improvements but still the problem persists.