2003 Ford Focus Engine hesitation

I had the compressor changed on my 2003 Ford Focus, DOHC 2.0L, 42,500 miles. 2 weeks later the car died and would not restart. Found half of serpentine belt separated and became chewed up, yanked bottom timing belt cover off and broke timing belt, due to wrong pulley used on compressor. Towed car back to first shop, which fixed A/C misalignment and replaced timing belt. Compression test found no loss of compression. Spark plugs were replaced at this time. After getting car back, car idles fine but hesitates from a stop and up hills. FOT bogs down until 3500 rpm, where accelerates quicker but in 3 or 4 jerky "steps". Car is much worse after warms up. Sometimes when gas pedal is quickly depressed in drive, I hear an initial small "pop" from the engine, kinda like a little backfire. Replaced spark plug wires and looked for vacuum leaks to no avail. Wonder if timing is incorrect but would expect problem to be more consistent. Any other things I can look at that they could have affected while replacing timing belt? Shop is being very difficult and not taking responsibility for broken timing belt, and does not want to recheck timing.