tach quits working in 4th gear?

[292farmer]

ok so i searched the forum tried some ideas and nothing seemed to work. in 1st 2nd the tach will work fine, occasionally in 3rd and all the time in 4th when i hit 2500-2600rpms the needle sticks, the motor keeps going up in rpms, but tach stays at 2500/2600rpms and wont move till i drop the rpms below that. the tach is a sunpro 2-5/8" the motor is a 292 stock ign. tranny is a sm465. what can be causing this all wires are tight, the switch on tach is at 6cyl, is this a tach problem or an problem at the ign/coil side i need to address? if its tach i could care less but if its an ign problem want to solve it. thanks everyone sorry for long post.