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Intermittent power loss Toyota v-6

My 1991 mini-motorhome, built on a Toyota pickup chassis with the v-6 engine & 5-speed recently developed a “hiccup” immediately after having the ignition distributor, rotor & wires replaced along with new belts, hoses, valve adjustment, etc. No work was done on the fuel system. Every once in a while (15 minutes to an hour or more, never more frequently) the engine will lose power for about 1/4 of a second, followed by a “thunk” as the slack in the drive train is taken up. It’s easy to ignore except when I’m laboring up a mountain in third gear when the “thunk” becomes an alarming “bang”. Other than that, it runs beautifully. Check engine light is not on. I haven’t been able to associate the hiccup with load, speed, temperature, or anything else. My mechanic has no idea what it could be, and it didn’t misbehave when he drove it. What’s going on?

Is this the original clutch?
Does the engine seem to rev when this happens?

The clutch is brand new, installed just before the problem started. The flywheel was machined. Works much smoother than before.
No, the engine doesn’t rev up, it LOSES power for about 1/4 second then comes back and makes a bang as it takes up the gear lash in the power train.