1990 Cherokee won't run over approx. 1500rpm

Almost giving up..

Renix ECU (replaced couple of years ago with Bendix)Multi point fuel
injection, 4.0 litre inline six with manual transmission, 110K miles.
It has been doing this over a year now,it feels like this Jeep has a
mind of it's own...sometimes good day,sometimes bad day (mostly bad
days recently).
Problem is, that sometimes it just dont run over 1500rpm, it doesnt
matter how much i press
the gas pedal,it just sputters and rpm gauge drops to zero,it does not
matter what gear i'm using,if i depress the gas pedal slightly then it
keeps going slowly and rpm gauge comes back alive.

But when it runs fine it really runs like it should (almost like a
sportscar)..

CPS sensor is brand new and giving nice pulses,O2 sensor is brand
new,TPS sensor is giving smooth voltage,coolant temperature sensor is
ok,manifold air temperature sensor is ok,MAP sensor seems to be ok,EGR
seems to work, stator in the distributor is giving nice pulses,fuel
pump ballast resistor is 1 ohm, (every sensor checked according to Jeep
Renix Fuel Injection Manual) i have checked the sensors using digital
multimeter (and oscilloscope for pulse giving sensors). I have done
measurements near to ECU connectors (by cutting/peeling the wires).
One strange thing is, that there is no noticeable difference if knock
sensor wires are connected or not.. and it it was also giving knock
pulses..more rpms,more and higher pulses. Basically my question is,what
should i try next...
could this problem be fuel related ?
what are the reasons which can make rpm gauge drop to zero?
For me it just seems to be electrical...i dont know anymore..

Please answer if someone have a clue about this.