You really need to check for a vac leak.Spray a can of brake clean around the intake and throttle body.Sometimes the throttle body base gaskets dry out and cause air leaks.Used to be pretty common on old TBI injected GM cars and trucks.

 

Replace the gasket regardless. It takes $2 and 10 minutes.

 

Which gaskets do I need to replace> There is one on top of the Throttle Body which attaches to the Air Filter. Otherwise Im lost here

 

I mean the gasket in between the throttle body and intake manifold. The one between the air cleaner and throttle body is important, but not quite as essential. It should be replaced too if its in poor condition.

 

It could be your vacuum booster or the check valve on the vacumm booster,try to have it checked,a vacuum leak can also make the oxygen sensors read lean or rich.