Layer gaps and shell issues!

[Mils24]

Hi guys

I am new to the forum and this is my first post. I have spent the last few minutes searching through for others who have posted about the same thing but cant find anything so apologies if I have missed threads and Im duplicating with this one. Any links to other threads is greatly appreciated!

I am new to 3D printing and recently received a Kickstarter Delta printer kit. I have it up and running and my first few prints have been pretty good. However over the past week or so I have started getting A LOT of missed layers / holes and gaps in layers as seen below or attached (new to embedding/attaching images too, sorry)

I have tried searching around various websites and forums for help and I just cant seem to get an answer for this.

Background info:
Heavy duty wooden frame Delta construction
High quality IGUS slider rails on each tower (nylon slider in alloy rail)
Carbon tubes with bonded in rod end bearings.
Models sliced in Cura
Board - Arduino Due c/w RADDS
Firmware - Retetier 0.92 (i think - 0.91 at earliest)
Printed in PLA
Heated bed - 65
Hot End - 210
Print speed - 50mm/s
Travel Speed - 150mm/s
Flow 100%
Retraction enabled
Nozzle size - 0.5mm
Perimeters - 1.0mm (2)
Top and Bottom infill - 1.0mm (2)
Infil 25%

Ive also tried various different PLA suppliers and all the same result. Note - the material is all kept in click seal bags with silica sachets in a box in a bookcase beside a radiator so I doubt moisture ingress to the PLA is the problem. I also have taken one of the spools I have in its bag and put it in the 'hotpress/airing cupboard' for a few days to drive out any moisture which didn't solve the problem.

I don't think it is software or firmware related as I have not changed any settings and have had prints that have had perfect shells without any problems shown above.

I am pulling my hair out trying to solve this and absolutely ANY help is greatly welcomed!!

Many thanks in advance