phase1
- identify the cause of the swings more than 3 and 1/3
It is limited to 3 and 1/3 diff change. But the new diff is calculated from the average diff over the previous 24 blocks. So you can't compare the diff to the previous block as displayed in the list. Please see https://github.com/nlgcoin/guldencoin/blob/master/src/main.cpp#L1286 and line 1299 and lines 1303-1306.

- work out a solution for point 1 first
We could change it so the diff is calculated only from the diff of the previous block.. Then the diff change between individual blocks could never exceed 3x or 0.33x. But that also means the diff can go x3^3 (x27 !!!) when there are 3 lucky/fast blocks. I don't think that is a smart thing to do.
- if the fix for point 1 is outside DGW code see if DGW is acting as expected in the first place
I don't think point 1 needs to be fixed. I'm not aware of any code influencing the difficulty outside DGW. DGW is acting as it should (as it is doing in other coins), but simply can't handle these hash/sec spikes.
I would love to see some input or simulation why this happens. Till now I'm not able to pinpoint the large swings we encounter. Your right it could be the large hash swings but why does NLG have more problems with that? So the more people look at the code from start to end and posibly come with a detailed reason the better.