For how many years? It could be that the 15A breaker is barely enough, and after 8 years of operating at, say, 14A, it is just worn out. The easiest thing to try first is replacing the 15A breaker with a new 15A.
Are these window units? Maybe you can find a higher efficiency AC to install upstairs that will use less amps for the same (or more) BTU/hr. You should be able to run a 12000 BTU unit on a 15A dedicated circuit if the EER is at least 10.0. You might also look for a 240V AC and switch it over to 240V.
Don't up the breaker to 20A unless you are *sure* all the wire is 12 gauge. But you might can make this work with 15A and 14 ga wire.
Bob