Tach (and engine) weirdness

[Runt]

Okay, I've got a coouple weird things going on, maybe related, maybe not.  I'd love to hear some ideas.
The tach in my 91 mk2 jetta (factory, w terminal)  seems to work really well sometimes, then not so well others.  Often, it will stop climbing, or even drop back down several hundred rpm, at or just over 3000 rpm.  Then it will bounce back up to engine rpm after a second or so, then sometimes but not always flutter up and down about 1000 rpm.  A couple of times it has done the same fluttering and bouncing throughout the rpm range, but then gone back to responding properly.  I have changed, tightened, and triple checked the belt, and I'm almost certain that the issue is not belt slip.
Also, at WOT, the motor itself usually, but not always, falls on it's face at right about 3000 rpm, then if given a chance will start pulling again and rev up to 4500 or so.  In high gears, it will accel/decel surge within the dead/low power spot, but will eventually climb through to pull a little better.  This surge seems to disappear if I back out of the throttle, but then I don't have the pull that I''m looking for.
I have quickly checked the alt connections and the stop solenoid on the injection pump, wondering if I was seeing a vibration in a connection, but did not see an issue.  Is there a connection somewhere else that these two wires share a plug?  Any other thoughts?  Any help is appreciated.

[maxfax]

I can't be of so much help on your tach, other than checking all the connections and grounds..  Not only at the alt, but also in the cluster..  I'm sure Vince will chime in with some more useful information..


AS far as your engine wierdness it sort of sounds like a fuel issue..  First and foremost, how is your fuel filter??  New? Kinda New? Antique?   The next thing I'd be looking for is an air leak in the fuel line to the pump..  It may not leak fuel anywhere but it could be sucking air at higher RPMs..  IF you still have clear line (that you can actually still see though) between the pump and filter, watch that for little bubbles..  You may have to rev the engine to see them..   IF you are pretty confident that you have no air leaks it may pay to check the pickup in the fuel tank..  It could be plugging with gunk.. 

A common practice is to connect an alternative source of CLEAN fuel directly to the pump and see how things work..  Since it sounds as though you you need to be driving the car and have it under load use appropriate cautions in doing so...

[macka]

It could also be a cracked water seperater. First thing I do is deep six that pile of junk and just run fuel lines to fill the gap left by removing the filter. Its located under rear passenger seat on the outside bottom of the car.