Slow to start but runs fine

99 Miata slow to start but once it starts it runs fine.
I recently had issues with the car not starting at all or making any sounds when I turned the key but that went away after about a week or two.
Then last week it wouldn't start at all without a jump, but all lights and interior accessories worked. It would run fine after it was jumped. I replaced the battery clamps to see if that was maybe the issue.
Now it starts but very slow and it chug-chug-chugs and when it finally turns over the rpm is high for about 30 seconds then goes back to normal. Alternator and battery are at 15 volts. Not sure what I should try first. HELP!