1996 Chevy Tahoe misfires

Ok here it goes. On my 1996 chevy tahoe it get a misfire code on the #7 cylinder. I changed the plugs wires cap and rotor to no fix. The car runs good when ur driving. But at a stop with your foot on the brake it is fine for a little bit then the RPMs start to go up and down and the car wants to move. And now after replacing all that it starts hard. When I turn the key the 1st time it wont start. It takes about 2 or 3 times before it will strart but after its started and I go to the store it will fire rite up. Not really sure what the problem is but im on a tight spending budget from being layed-off. Please help