1000 watt microwave trips 20 amp dedicated circuit does not trip all the time I am thinking heat build up over time weakening breaker should I replace breaker or possible upgrade wire size to 10 g

A 1000 Watt microwave should not be tripping a breaker on a 20 amp dedicated circuit. You could have a defective breaker, or a defective microwave.

I'm betting on the microwave tripping the breaker.
Do some searching and try to find if that microwave is under any recalls.

Using Ohms law (Watts/voltage=amps) you microwave when in use is only pulling 8 & 1/3 amps. The should have zero effect on a 20 amp CB that is working properly.

Are you certain that this is a dedicated circuit (20 amp CB inside panelboard connected to the microwave receptacle using 12 AWG copper conductors) and nothing else. No other receptacle outlets or lights?

I would swap the breaker with another 20 amp breaker from the panelboard and see if the failure move with the breaker. If it does, replace the failing breaker.

The reason I would swap the breaker first is cost. The old breaker would/should be replaced with an AFCI (Arc Fault Circuit interrupter) type breaker and those are currently fairly expensive.

Any other 20A line in the house to try the microwave on first?