Auto parts store said my battery was ok but when.

Generator is quitting. The "battery" warning light means the generator is not recharging the battery while you're driving. Have the charging system load-tested but since you have an intermittent problem, that testing must be done while the problem is occurring to be valid. You can do a preliminary test too with an inexpensive digital voltmeter. With the engine running, the battery voltage must be between 13.75 and 14.75 volts. If the generator quits working, the voltage will be closer to 12.6 volts or less.