Engine misfire, loss of power and shuts off

The vehicle runs a little smooth when idling, but the engine misfires very bad when load is applied i.E, A/C on or gear is engaged, it shuts off while driving, at times it hesitates when accelerating, sometimes it drives nicely that you can drive 5 kilometers without having any of the problems mentioned above, when it shuts off while driving it starts right back up and drive for a kilometer or 2 then shuts down and start again, sometimes it hesitates to start but eventually starts back up.

History of the car: last year in May we changed the valves as it had broken most of them, this was because of a mis-timing after changing the crack shaft pulley, it was correctly timed after changing the valves and worked just fine, In October the transmission got damaged and the car was parked until January this year, we changed the transmission 2 weeks ago, the transmission runs smooth and changes gears without any issues.

The engine misfire and loss of power started after the car got started after being parked for a few months, I was told to replace the fuel pump and filter which was done, I replaced the spark plugs but this has not solved the problem. The car does not have an engine light on, a diagnosis machine was connected to the car and did not bring any codes or faults.

The mechanic insists that this is a mechanical problem and that there are some valves that might not be closing completely hence the misfire and loss of power.

The mechanic says we need to re-time (timing) valves again. I am a little hesitant to go along with this. My vehicle uses a timing chain (not timing belt).

Could the mechanic be right and re-timing will solve the problem or what could really be the problem causing the engine misfire and loss of power?

Your quick response will be highly appreciated

Thank you,
Kaoma.