CZ 83-380 feeding problem

I have a CZ 83 I bought from Cole Distributors (it is probably a Polish police trade in) that I am having a feeding problem with. I have the original mag, as well as 2 aftermarket mags, and am having the same problem with all three. It will feed and fire several rounds then the slide will lock open with a round sitting at the top of the mag. I can hit the slide release, or rack the slide and the round will go into firing position, the slide will close and I can fire several more rounds before it happens again. There doesn't appear to be any problem with ejecting spent brass.
Can anyone give me any ideas what the problem may be? Any ideas will be appreciated.