reluctant starting - electrical

[ChrisG]

OK
Had the problem to some degree last year- and just been to start the 80 Sport that has been under wraps in its garage since mid December.

i)Connected battery, new last summer - voltmeter reading about 12+ volts.
ii)Turned key, engine turns over, almost at 'normal' cranking speed then, within a couple of seconds slows down - as though the battery is flat. At this point the volts are reading low.... possibly 8-9 volts.
iii)solenoid is then clicking in and out furiously
iv) connect jump leads, engine turns over, normal cranking speed, and after about 5 secs engine fires up.

Once running you can turn it off and restart on the battery, after a few minutes, or several hours.

This was a problem earlier last year and as the battery was 10 years old I treated it to a new one --so symptoms have been similar with two batteries , an old and a new.

Possible that this is a faulty battery??? (Seems to hold its charge ok).

Bad electrical connection somewhere between battery,solenoid,starter ???
Seems to have started, possibly, after new gearbox mounts were fitted - coincidence??

As always, thanks in advance for suggestions

oh and I reeaally enjoyed the 15 miles I did in it this afternoon....