1999 Other GMC Models please help no one can figure this out

chevy suburban battery light and air bag light comes on every so often and then i start to (jump) serge down the road for about a mile or sometimes my more. Then the light go off and the things drives fine for about another 50 miles and then does it again. sometimes sooner and sometimes less but it seems to be getting worse (serging) when it does happen. any ideas what is going on? it has been hooked up to computer and all reads fine.