My 02 Chevy Malibu motor won't turn over

OK so lately when the car and heater are on
together, the engine will briefly lose power
for a cpl seconds on and off. The it began
having diferculties starting. Just now had to
hold the key for almost a full ten seconds
before it would crank. Been a few times
lately we've had to try it a few times before it
would actually start. But this last one was
the worst. It tries to turn over but fails.
Eventually it cranked but each time we start
it its been getting progressively worse. Not
sure is this matter but we have had
diferculties turning the key since we bought
the car last month, but after jiggling the key
it eventually turns please help!!!