Check for a carbon fouled spark plug and or bad plug wires. Very common for a miss under load. Even if the wires and plugs are relatively new it happens more than a person wants to think. Sure there are a lot of possibilities but that is where I would start.
If you have just done something requiring that the plug wires be removed make sure that 5 * 7 are not reversed on the plugs. Very easy to do and makes for a very light miss till under load being they are so close together.