misfiring? other possible problems?

[SVXgarcia]

Ok here's the deal. Nothing was wrong with the engine until it was switched over to another SVX. Everything worked well no loss of power or misfiring.
The timing, valve cover gaskets, main seals were replaced before putting the engine back in the car

Once the engine was finally on the other SVX there seems to be a misfiring or loss of power only during take off. When idling the car wants to stall but It doesn't. I have changed the spark plugs but no help. No CEL lights or anything else.
What could have gone wrong? If everything was fine until the engine was switched.