P0173/P0174

I have been fighting PO173/174, so far I have replaced the Mass Air Flow meter, after cleaning several times, that code was cleared. I installed a new air filter, new main fuel filter. I checked all over the intake manifold several times checking for a vacuum leak with no luck. I even used engine start fluid, still no luck. I took my car to a Jag Indy, to have it smoked tested. While there I asked him to look at an oil leak. During the smoke test, he said smoke was escaping from around the timing cover. Could this possibly cause PO173/PO174? I was leaning toward replacing the MAP sensor. I also bought a new PCV and hose. I am beginning to feel like I am troubleshooting by replacing parts. I read the forums to gain insight to better diagnose the fault. Right now my car stumbles between 1500-2000 RPM, mostly when under a load such as going up a small hill or any incline for the matter. I sealed the air box and intake tube, still stumbles around 1500-2000 RPM. My Indy wants $1,100 to replace timing cover gaskets and associated seals. To address the timing cover leak, I put in Lucas oil stop leak, it worked, no more timing cover leak. Still the motor stumbles at 1500-2000 RPM. The car has 132K, on odometer and anything past 2000 RPM, it pulls very hard. When it does stumble, I do not always get a CEL, any help or advice is greatly appreciated.
Happy Holidays. John