The timing belt (cam belt) connects the camshaft(s) to the crankshaft, and
maintains a spatial relationship between the tops of the pistons and the
bottoms of the valves.  When the timing belt breaks, this spatial
relationship is dissolved, and shortly thereafter, so are the pistons and
valves.  Mechanics call this "serious motor damage".

Mitsubishi recommends a new one every sixty thousand miles.  I just had it
replaced in my Diamonte.  The mechanic said it was original equipment, and
the car had 112,xxx miles on it then.  It looked like it was in good shape
to me, but I wanted the peace of mind of knowing it wasn't going to
immanently fail and leave me stranded with a seriously damaged motor.

I had a complete tune-up, replaced the accessory and timing belts, the
water pump, plug wires and plugs, cap, rotor, etc., and spent about $1350
on it all.  And yes, that is a lot of money, but I'm confident that with
regular maintenance it will run another hundred thousand miles.

I'd recommend getting the timing belt professionally assessed.  The problem
with that is that by the time they tear the motor down far enough to do
that, you might as well just go all the way and replace the thing.

What you might want to do is get the symptom (runs rough) professionally
evaluated.  A mechanic ought to be able to tell you what is wrong, and give
you an estimate for how much it will cost to fix.  It might not be as bad
as you think.