Should I(t) stay or should I(t) go. (In the words of The Clash)

[ian.sandom]

I have a dilemma. I have a 54 plate 260 which I bought for £18,000 in June 2005. It's the most I have ever paid for a car and probably ever will again. I had an LPG conversion fitted in April 2006 and intended to keep this car for many years and not waste any more money on changing cars every year or so like I had done in the past.
It has also been the most disappointing car I have owned. In the first week the fuel filter came apart, then it went back for SRS faults, gearbox oil seal, fuel pump fuse kept blowing, the tyres wear out in 5000 miles, it only does 15 mpg (Thank god for the lpg), most recently I was off the road for 11 days trying to source a starter motor (luckily "scooter" came to the rescue).
I still love the car but have started asking myself whether I can live with such an exclusive car on a daily basis. I know a lot of you have 260's as weekend or hobby cars but I run mine as my daily transport and being an HGV lorry driver have to leave home at 4 or 5 in the morning when there is no alternative transport if my car won't start or get me to work.
My dilemma is this: Do I get rid of the 260 whilst I can still get between 3 and 5 grand for it as a trade in or do I run it until I can't get a part for it and then scrap it?