Sounds like a burr somewhere inside. Have you had it apart? If not, anything on the hammer where it looks to be rubbing? Does it do it with the grips off?

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Also make sure the adjustable trigger stop (I think the 17 has one - it's the little lever-looking thingie in the back of the trigger guard) has not gotten out of adjustment. Loosen the stop and push it all the way back and see if that stops the glitch. If it does, the trigger stop has moved too far forward and is not giving a clean release.

I would pull the side plate off and cycle the action while watching the parts move. It could be as simple as some dirt in the action, or as complicated as a broken single action sear face. It could be some old oil or grease gumming up the action, something out of place, etc. The only way to tell would be to pull the side plate and observe it. If you are not comfortable pulling the side plate, by all means, find an honest, competent 'smith to look at it. It is much better to pay someone for their expertise and time, than to ruin an otherwise good gun by improperly disassembling or reassembling it.

I've been out of pocket for four days so just saw your post. First I would see if I could get my hands on a Jerry Kuhnhausen manual on Smith & Wessons. If not, go on line and look at some tutorials on disassembling a K-frame. Nearly all Smith & Wessons are similar so once you can disassemble one you are pretty well prepared for most of their revolvers you encounter. I would take it apart. Clean it. Get rid of all the old grease and oil. Use a very light lube on all the friction points but only enough so it isn't completely dry. Reassemble it. If the problem still exists it is a mechanical problem and you need a gunsmith. Most of the problems with these guns are a result of someone attempting to fix what isn't broke. I'm not saying don't attempt to fix you own gun. I am saying if you attempt it, do your homework first. Good luck, you have one of the best guns Smith & Wesson ever produced.