No, the info in your original post want enough for even an experienced electronics expert.
When you said "5V led strip", my best guess was that you were using ws2812b strips. These are individually addressable rgb 5V strips. A low value resistor is normally used on the data line.
How much current do your strips draw? Have you measured the voltages as I suggested? What is the wattage of those 270R resistors?
I guess we disagree then Paul. I am no electronics expert by a long long way, but I may have had enough info in an example like this where the OP was asking for a simple pointer, rather than a detailed electronic engineering illustrative answer, to suggest what I might have thought to be the issue. We're all different I guess!
Oh and if I was using addressable LED strips, given that they are far more complex than simple LED's or LED strips, quite simply, I'd have said so, Paul! Another current project of mine is to add coloured LED strips to the stair hand rail in my night lighting rig and I am finding writing the code for these complicated enough - God help me when I move on to addressable ones!!! eeeek!
As yet I've not measured the current, neither have I done any testing or measurements in-situ in the circuit, I've simply removed the LED strip and tested that on its own, and it appears to be OK. I'll do as you suggest though, and will measure voltage and current tomorrow to see if that gives me any pointers. I'll report findings here.
Re the wattage of the resistors, I'm afraid my inexperience and lack of knowledge again rears its head here as I am not sure. They came with an Arduino starter kit I purchased a few weeks ago and the wattage isn't given. So unless there's a way similar to coloured resistance markings, I have no idea how to ascertain the power rating of them! I'll do some googling and see if I can find out how to obtain the power rating. Personally though I'd say it is pretty low, as they are physically quite small resistors, and thin too, as are the wires on them!
Computer power supplies often have trouble supplying 5v when the 12v rail is not under load - they're designed to supply something that uses both 5v and 12v (with a heavy load on 12v), and don't always do well with loading on 5v but not 12v (because they cut corners for cost savings - based on assumptions about the type of load they are expecting).
Better to use phone chargers and butchered USB cables (or purpose built 5v supplies) to power 5v projects.
That's an interesting point re ATX converted PSU's, I wasn't aware of that, thanks! I do have a small load permanently across the 12V supply - currently (excuse the pun!!) using a 20W car headlamp bulb!
I might try switching the PSU to a USB one to see if that changes anything.