[strike]IG cassette teeth are thicker, so they need chains that are wide enough to fit -- unless noted, a regular HG chain may not work. According to Sheldon, the SRAM PC-870 chain (eight-speed) works with IG cassettes.[/strike]

Oh wait. Your chainring is marked to use IG chains because it was installed as part of a complete IG system, and it's a more visible place to put the reminder. AFAIK, there's nothing otherwise special about it, and you should be free to use whatever chain you want, provided it's not worn out.

Since you installed the cassette and chain (right?), the shop wouldn't know if the derailleur is out of whack, so unless you're good at it, it may be a good idea to have them check it out. Take a picture of the chainring for us, if you think it might be that.

P.S. I'd be interested to hear how you like the HG41-7 cassette when it's all sorted out. I was surprised/pleased to see that Shimano had added a new silver-colored cassette to their 7-speed range.