Well for a start, whats it doing and what are the 'problems'?
There is no way in hell i would let a taxi depot anywhere near my car! Your best bet is to find a decent mechanic who can do a full service on it. It may not be cheap, but it is definately worth doing if yuo want to keep your car in good order, and you'll usually find the more business you give one mechanic, the better he will look after yuo
Have a bit of a look around the forums, because a lot of common problems are discussed and you'll be able to find the answer to them too.