Battery voltage drop when starting

[Horizon]

Hi Guys
i fitted an aftermarket chinese headunit last April. Recently it has been playing up every few starts by not booting up properly and locking itself up. The first time it happened I pulled the headunit out and separated the quadlock, checked for signs of a bad connection/ dirt or a pin pushed out, nothing out of the ordinary.
when I reconnected the quadlock the radio booted up. A few days later the same thing happened , this time I pulled the radio fuse, waited a few seconds then refitted the fuse, same again the headunit rebooted and was fine for a week or so.
i was sick of pulling the fuse, so I fitted a toggle switch in series with the yellow permanent live feed to the radio, each time it played up a quick flick of the switch and back to normal. I suspected it could be a dicky canbus module supplied with the unit, not always powering up the Accesories red wire, so I ran the ACC wire to a switched live in the fuse box using a fuse tap. ( now reconnected to the canbus module )
Problem is now almost daily and I have noticed if I wait until it has booted up the start the car the screen goes blank and it tries to reboot again, I left the battery on charge over night, still the same. Suspect battery maybe, as it's coming up to 5 years old, hardly ever gets used in the week, proper battery killing lifestyle . Took it down to my local battery shop he drop tested it twice, dropped to 9 volts then stabilised. He said " Nowt wrong with that "
returned home put my volt meter on it and it drops to 8/9 v when cranking. Nicked the battery off my 62 plate Caddy and swapped them over. Headunit now doesn't drop out when starting. I will try it over a couple of weeks to test, then if it's sorted, purchase a new battery. ( Caddy is my firms van )
Any input would be welcome regarding battery types and if the voltage drop could be my problem
thanks in advance, and sorry for the long winded tale.

[SammoVWT]

It may be fine to crank, but its draining all the power out of the rest of the system. With a battery that old and dropping by that many volts I'd be tempted to replace it. Whats the voltage like with the car off across the battery? Less than 12V?

If the volts drop a lot, it means is struggling to provide current to the starter etc.

Do you get any odd noises when cranking? No buzzing or grinding? Dimming lights?

[Horizon]

I'm getting 12v with engine off, 13/14 v engine running. I will monitor how it behaves with my Caddy battery on over the next couple of weeks, then if it's fine I will get a 60 ah battery from ECP, only about £60. Thanks for the reply