5m is quite long, perhaps longer than necessary. Or maybe your coil is too narrow or kinked which is making it hard to push the wort through.
It's 8mm bore so quite narrow (cheaper option than 10mm) there is only one or two places where the coil has kinked, thought I'd done a good job coiling it around a wd40 can, I think I have some coil left so might look to make it again at say 3m long?
Another possibility is that your grain bed is getting compacted - Only tested with cold water so far, so no grain bed so far just trying to go from HLT to mash tun through the herms coil with cold water.
I reckon the 14W solar pump at 5m head should be good enough. so far I've spent around £45 on pumps and would need another £35 ish inc postage for the 14w 5m head one and if still not reliable would still need another perhaps... On the same token spend lots on a pump like the march nano and have similar issues as the max head is 6m however I suspect at 0.125 HP 230v at 60 herzt pump would perform better than the 14w solar pump? 17gpm flow rate at 1ft and 27ft max head.
The thing I don't get though is it seem to work for a few minuets doesn't seem to be struggling or making any noise or smell or feel hot but then just stops, so then I turn it off leave it a few minuets and it starts to work again and then stops again...
You'd think they'd take it off their website then - you would, unless they are exporting or something? Interestingly some of the American companies will allow me to import them in, not sure of what the issue would be though with it not being CE certified? I have also been looking at more beer pumps either the chugger or the march AC-5B-MD Beer Pump which is £212 + Postage and any duty!
I know someone awhile ago said about the cornlius flow pump that was about £70 but not sure on spec (the only ones I've seen are for fermented wort as low temp threshhold)