"some" problems?  Would depend upon the issues.  Wife drives an 02 Optima,
although a 6 cylinder, and had "some" problems with it when under warranty.
We had to replace several window regulators, a fuel cap, a broken radiator
hose fitting and a computer issue.  All this under warranty.  The amount &
frequency of the problems were consistent with many other cars I've had over
the past five decades regardless of the cost of the car - beater vs luxury.
Our bumper to bumper expired at 70,000.  A few weeks ago, we had the first
ever out of pocket expense for the car (other than preventative maintenance)
at 102,000 miles.  Had a faulty sensor making the check engine light go on
intermittently.  All in all, we are very pleased with the car.  Mile for
mile and dollar for dollar, it has been the most economical car we have ever
owned.

Rich

I can't help you with an older model.

But if you have any doubts, trade it.  The Optima was re-designed for '06.
Even if there were some initial teething problems (I haven't heard of any),
by '08 they have this down.

And the '08 Optima is a seriously good automobile.  The new 4-cylinder
engine has power that makes the V6 virtually unnecessary. The new 4 also has
a timing chain instead of a belt, getting rid of that occasional maintenance
nuisance.

And with the 100,000 mile powertrain warranty, if is hard to go wrong.  Add
in the relatively low price of the Optima compared to other cars its size,
and it is one of the best values on the road.

Finally, at least in my area, you can a $500 rebate, plus a $1500 loyalty
bonus for going from a Kia to a Kia.  If you can find and choose one of the
few leftover '07's, the rebate goes up to $2500.

Hope this helps.