This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
It may be that your ZipGS card is not making good contact in its Slot socket. This is a fairly nasty problem which has led users to pursue a number of false cures. With power OFF, pull your ZipGS board and inspect the bottom-of-card connectors. What you will, most likely, notice is that the connector traces end approximately 1/8 inch or so from the bottom of the card. Unfortunately, the GS Slot sockets make contact rather near the bottom of cards plugged into them-- roughly 1/8 inch or so from the bottom. The reason your GS sometimes hangs is that, sometimes, the ZipGS card is not making good contact with all Slot connectors. One 'cure' is to make sure the ZipGS card's contacts are clean and that the card is thoroughly plugged in-- i.e. well lined-up with Slot contacts and inserted as far as it will go into the Slot socket. A mildly tricky additional step is to use a small jeweller's screwdriver to reach into the Slot socket and carefully twist/pull/bend-out each contact very slightly (naturally, with power OFF). You do not want any contact to normally touch a contact across from it. Whatever, if your GS starts okay and does not exhibit the same kind of hanging in the future, you know that the ZipGS card is well socketed. A more permanent, reliable cure is one not recommended for someone without experience working on circuit boards: You find a better Slot connector socket-- one with gold contacts which touch plugged-in cards higher up and with circuit board connections which will fit into the original holes-- and replace the connector. This is a very tricky replacement which requires careful de- soldering of the original Slot socket, cleaning of contact holes, and soldering-on the new socket-- all without burning th e circuit board or slicing traces on either side. Actually, slicing traces is okay, IF you are prepared to repair the damage. (Yes; I did this replacement on our GS. It works; but, If I had known what a hassle it would be, I probably would never have done it!) ----------------------------