1999 bora starting headache

[boxofmatches]

I have a 99 bora 1.9 tdi 90bhp, about four months ago we stopped at a shop for 5mins, retured to the car and it would not start, after about an hour the car started again, this has happened a few times since, now over the past few weeks the problem has got worse, the battery is strong enough, I have had it tested twice now, the engine turns over but just wont start, now here is the odd part, if I get jump leads and hook up to another vehicle then the bora will start, just today while at the local quickfit having the battery tested the car failed to start, the garage guy then went and got a 240v booster to start the car although the battery was strong and not lacking power, well the car did not start, then my Wife pulled out the jump leads and hooked to another car and the bora started on the first turn, a few months ago this again happened and I tried to start the thing for about an hour, this was before we realised that hooking up with jumpers would start the car, I then left the car for the evening and went back to it in the morning and the car started first time.
Any help greatly appreciated,
Regards,
James