Engine running VERY LEAN

That's one sign of a hefty vacuum leak. Might be a vacuum hose, gasket, or the EGR sticking open. Just have a mechanic find it.

If the head gaskets have not been done yet I'd check that first. Are you getting a lean trouble code? Pull the spark plugs and look at the tips. The ones that are white are burning lean. If it's all of them it's most likely a fuel delivery problem. A vacuum leak will also usually manifest itself as a high engine idle. Jim

Check fuel pressure at fuel rail. watch for pressure drop when you try to drive. If it drops out it can be the fuel pump or filter.

Is the catalytic convertor plugged up? Is the check engine light on?