Continuous AF Problem

I recently sent my D300 to El Segundo for general service because it is more than two years old, and my percentage of good photos using continuous AF seemed to be declining. Now, after spending $250, I can't buy a decently focused BIF shot. I have tried for three weekends, and I have one moderately good shot to show for my trouble. Yesterday I took my D700 along for a comparison, and there IS NO comparison. The D700 gets crisper shots with better detail at the same distance, even though it's almost like a wide angle shot with that big sensor. This should not happen. The D300 should focus every bit as well, and I should get a lot more detail, but it just ain't happening. The camera is fine in AF-S mode, but I can only achieve good results in AF-C if I put it on a tripod and shoot a large still object. I've tried many combinations of Dynamic area AF and Focus lock, and I always shoot with "Focus" priority. When I compared the two cameras it was with 21 point, Focus lock "Short". I used the same Nikkor 300mm f4 and TC14EII on both cameras. For a "final straw" attempt this morning I used a rocket blower to clean the mirror chamber then opened the mirror to blow off the sensor. I finished with a "Sensor Clean" for good measure, but it doesn't seem to be one bit better.
I am sending it back to El Segundo tomorrow, but I was wondering if anyone else has encountered this problem. Maybe I'm missing something.

Did you use the same lens on both cameras? Have you tried using different lenses on the d300?

Same lens on both cameras with the same teleconverter. 300mm f4 with tc14eii. Tried a 70-200 for a few minutes on the D300 but it wasn't any better, with or without VR.

It’s been awhile since your last post. I’m wondering what happened? I had singular problem with El Segundo. After 4 times to the shop they finally got it right, they tried to say I was doing something to it and all kinds of stupid stuff. It took a general manager to make them fix it. 4th time before they cleaned it right! I have over 10,000 shots after the general manager made them fix it right and it is still focusing great. I’ll have to send mine in again next year for service but unless they increase the qualifications on the techs at El Segundo I think I’ll send it somewhere else. I’ll be watching for your follow up post, thanks