98 XJ--Wife goes to leave work and wont crank over selenoid clattering. Co- worker jumps it and once started ,revs to 3,000 rpm and stays there. Told her to shut it off and wait for me.I jump it and starts normal and idle rpm is fine. I drive it home and intermittently the volt meter pegs over to 19 volts and check gauges light comes on. After a few minutes returns to 14 volts and check gauge light goes out, then after a few minutes it does the high voltage check gauges light again.When I get home voltage had returned to normal.Have not done any troubleshooting yet. No throttle issues on the drive home. Tried to crank after the 1/2 hour trip home and have low voltage ,wont turn over. The battery is only a few year old Optima and don't think it is a battery issue.Alternator or regulator or both? Would a bad regulator cause the initial battery drainage? Any help would be appreciated.