There's a 2-wire connector on the side of the distributor. (it's used to de-energize the coil when doing a compression check) Check it for a bad connection. If it's clean, there's a good chance your coil or ignitor is going out. (my tach did the same thing: drop to zero, bounce back up to normal, intermittently at first; then it got worse mega bog at any speed)

Take Care,
ChrisS