Engine stumbles when accelerating

1991 Mazda B-2200 manual trans 126K miles,runs okay until engine gets hot,then has two definite dead spots in RPM’s when accelerating:approx.2600 and 2800 RPM.Dead spots are consistent and do not vary up or down.They also do not show up every time.Lately I detect a loss of power in those two zones when driving on highway.accelerate or slow down gets out of the 'dead spots’and normal power.Also has a hesitation fron minimal throttle to medium or greater throttle.Any help?

Any history on the car? Has scheduled maintenance been done, other problems?