Got a used Colt 1911 80's model .45ACP. The slide locks back as if empty about every second or third round. This is with four different magazines, each bought separately and different makers!
Any suggestions on fixing this problem?

your recoil spring is to light for the loads

Will replace that immediately to see if that helps. Thanks,
Max

The most likely culprit is your bullet noses hitting the slide stop.

1) wrong slide stop. If its other than a .45 the lobe may be protruding too far into the magazine channel.

2) your ammo. If its reloads you may need to shorten them up.

Generally the spring comes into play when the slide won't lock back on the last round. A proper 1911 will feed any decent ammo from any decent magazine.