If it is only doing it under a load or acceleration, I wouldn`t think it would be timing off. If it were the timing, it would even do it just idling. At least that is my experience with the off timing in the past.
Are you sure all the wires on plugs, etc are pushed all the way on? And anything else that you were working around as well? Just a suggestion, I try to do the simplest and easiest route first.
If it were me, I would do the compression check, maybe you are looking at a head gasket leak, issue?
Hope this helps, but I am sure you will find the problem shortly.
~Judy in MI~