valve clearance

My first time here...I read many good advisement's, so please help figure out some repair priorities. My 2005 no frills 2WD with 149K has got a leaky valve cover and my mechanic said I should replace the gasket. While he's there should I also... replace the timing chain and tension-er, (the...

Ok, here is what happened. My first problem was rough idle, engine stalling after warming up and Check engine light on. Codes were P0339 and P0172. The repair was Idle and CPK relearn procedure. No change in problem. Second time, check engine light on again, codes were P0304 and DTC 0172...