Unusual starting issue from a cold start

I have started buying up and repairing the older, prettier (in my opinion) white G5 iMacs without any official training just playing really and I have, what I think is an unusual issue that i can't fathom out so if anyone has any suggestions feel free to answer.

So I had this when it wasn't booting at all, replaced the logic board and all was well. Restarted lots of times and no issues. Left it a few days as was working on another 17" Intel model and when I went back to it it started, light, chime and then switched itself off.

I have been swapping bits and pieces across machines but the same issue every time from a cold start.

I have no idea why I did this but removed the RAM and let fans etc run for about 5 minutes and it warmed up nicely.

Replaced RAM and it started 1st time properly.

I have tried this half a dozen times and it works every time so why will it start when 'warmed up' but not from a cold start?