1997 Chevy S-10 Check light on

Been having problems with my truck wanting to stall almost when I am sitting at a light or something and it just does not really want to go. I have had it put on the machine 2 times and it says my truck is running lean but can't seem to find out why it is doing this. It is like it is losing power and then speeds back up...goes up and down like this when I am sitting still. When this is happening the gas gauge moves up and then when it speeds back up the gas gauge goes back down.