Speed faults and an RA82 disk

My PDP-11/84 came with an RA82 disk and after three years of work I have it finally coming up to speed and going ready. I had to redesign the circuit that generated a tach pulse for monitoring the speed of the spindle. The original circuit never wanted to work for me. http://i11.photobucket.com/albums/a166/ballsandy/Computer%20related/logic.jp g http://i11.photobucket.com/albums/a166/ballsandy/Computer%20related/IMG_5731 .jpg The new circuit however is unreliable. At complete random the drive will throw both nonfatal and fatal speed errors however at no time can you hear that the speed of the drive has changed (and for error DA and error 26 you would definately hear it), nor can I catch the last moments before spindown on an oscilloscope as to see if the transistor is locking up or if there's some sort of interference. Again, it occurs randomly. I can be seeking or reading or writing or just idling the drive and it will happen and it makes the drive impossible to use. Could this just be me using too low speed a transistor or could this be something else that I have not yet considered? The reason I'm bypassing the original 74LS14 is that even when replaced it leaks about a volt onto the input pin and forced the signal high. I could not figure out a way to pull that down and still get the signal to come up properly so I simply removed it from the equation. -John