misfire on decel

[kytaylor21]

2011 yukon Denali XL, 6.2, 100k miles. Lifter starts to chatter, rather than $3500 for new lifter job I purchased a NEW engine from Jasper engines. Mechanic instals new engine I drive 20 miles the check engine light comes on  10 seconds and turns off. I check code PO300 and PO305 (misfire) I swap plug and spark plug wire  from cyl 5 (po305 is cyl 5 misfire) to cyl 1, still reading po305, I swap coil pack from cyl 5 to cyl 7 and I swap injector from cyl 5 to cyl 3 still getting po300 and po305 I take to mechanic after working and troubleshooting all day he swaps plug wire on cyl 5 with my original plug wire (from previous engine) and for the first time there is no misfire, I begin driving home and 5 miles later check engin back on????? any suggestions? possible valve out of adjustment? according to snap-on scanner it only misfires on decel. car runs great, no rough idle, no lack of power. Any help would me much appreciated

[davester]

Hydraulic lifters, so no adjustment, only replacement if they fail (and they generally fail so the valve doesn't open or only opens a little).