Thanks guys for all the contributions, let me tell you where I'm at now.
I have taken her apart completely and re-assembled with all the alu upgrades which include, swashplate, alu lower rotor hub, longer inner shaft with alu upper rotor assembly (I'm getting so good at stripping this thing down and re-building I can practically do it with my eyes closed!)
Stock e-flite blades.
Batteries are brand new and include both the stock e-flite battery but also the new Flight Power Evo battery.
Flew her just now and from a freshly charged battery she flew like normal for all of 5-6 minutes then lost altitude and hovered just a few inches off the floor. No sound of loss of power from motors or anything like that, ergo batteries are ok!
On full throttle she still continued to hover a few inches off the deck but failed to gain any altitude.
Landed and rested for couple of minutes after which she flies again like normal but for less then a minute then back to hovering just inches off the deck. Repeat the rest period of a couple of minutes and she takes off again like normal.
I'm totally bamboozled.
Also, have changed blades repeatedly so don't reckon it's in the blades, hence the reason I come back to the 4 in 1 unit. Could this be past it's sell-by date and perhaps overheating and not allowing sufficient current to pass to the motors (I'm no electronics expert by the way) and since I've tested/renewed just about everything else this is the only conclusion I can draw.
What do you think?