What's wrong with my car?

The other day, I bought myself a 1991 Holden Apollo (2 liter) for $900. After my mate drove it around and installed house speakers in the back, it refused to start. Now, it sometimes clicks, it sometimes turns a couple of times, & it sometimes tries to turn but stops. I had two mechanically minded blokes look at it, but they couldn't find the problem. they say it's either the battery (voltmeter says it's fine), spark leads & plugs or the starter motor. Can anyone help me? this is my first car, & it was running so well before it suddenly started doing this.