power loss

[T3NSION]

About 2 weeks ago I had the timing chain, upper guides and tensioner done on my 89 420 SEL.. I also had the transmission and torque converter drained, filter replaced and fluid changed. I picked up the car and everything was fine for the next few days, but I noticed oil leaking and when it got to running temperature it would smoke, of course, because oil was leaking onto the exhaust system. I took the car back into my mechanic and it was simply one of the valve cover gaskets had not seated properly. Simple enough, he took it off, cleaned and redid it, topped up the oil and cleaned my engine. Now all of a sudden my car feels like it is running at 40-50% power!!!! It takes longer to get up to speed and once I am at `in town` types of speed, I can push the pedal to the floor and quite literally nothing happens!!! It is as if the throttle is no longer in adjustment or something is preventing it from opening all the way. Could it be the plug wires were not replaced in the proper sequence??? Any ideas of what I might be able to check myself before wasting another 2 hours waiting at the garage for someone to look at it?

Thanks,
Kyle