So something has degraded over six years.
Here's one generic troubleshooting strategy for this problem:
If you can't eliminate the compressor motor as a suspect, time to call for help.
Unpower the unit.
You can do this elimination by replacing the six year old compressor motor start capacitor
or by first checking the existing cap with a DVM, 9v battery and a 50 cent 1 megohm resistor from Radio Shack.
Caps should first be discharged before testing unless they already have a resistor wired across them.
If the symptoms don't substantially change with a new cap, or if the old cap checks good, I'd say it's the compressor.
If a new cap does change the symptoms, for a few more $5's you may want to put a hard start kit in anyway, to take the burden of handling the normally high compressor startup current, from the circuit breaker.
HVAC people on this forum seem to like them, but I'd only put it in once you make sure there is a good cap in there. I think the kit is a help but not intended as a cure for other bad components.