It's much better if you can show a stable earnings and savings record. If you buy and sell cars and your account goes from overdrawn to over £4000 in a short period of time, it can look a bit suspicious and the ECO might think that you have borrowed the money just so that your account will look healthy when you apply for the visa.
What the ECO's like to see is a reasonably healthy balance (usually around, but not necessarily as much as £2000) over a sustained period of time and with regular amounts going into your account each month. This will show him or her that you have disposable income at the end of every month after you've paid all of your bills etc.
Having said that, you may be able to explain to the satisfaction of the ECO, the erratic changes in your saving in the letter of support that you include with the application.
Iain.