Is it probably the ECU?

Where are your timings at. A lazy idle timing will do all that.
Reverse polarity is easy to spot with a timing lite; the rate of advance goes bonkers instead of its usual steady rate of increase. To reverse the polarity, the connection between the dizzy and the ECU has to be flipped, either with an oem jumper(I have one) or an alternate mag pick-up(different colored leads) or by cut and splice.