I have an 05 Ford Mustang 4.0 V6 Auto. About 3 weeks ago I had head gasket replaced on drivers side and since then I have a misfire on cylinder 5 and that cylinder only has about 60 lbs of pressure it goes up and down sometimes it idles good and it starts idling rough again. What could be causing this? Did the mechanic do something wrong ? Please help

 

Botched head gasket job, you should have a lot more than 60psi of compression if that's what your talking about.