I have a 1992 Nissian pickup 4 cyl that idles high at 2000 rpm then drops to around 600 and back up again. This happens every 2 seconds. Also at any speed the truck shudders like it had a dead miss or like it is running out of fuel but it does not do it in a set pattern. What could it be? vacume leak?

 

This morning I checked for vacume leak using soapy water could not find any. also pulled the PCV valve and it rattles and is functioning properly, also checked the EGR valve and it is also ok. Any other ideas?

 

vacuum leak is always a possibility....(internal vacuum leaks are TUFFFF to find)..
But I'd bet on a faulty MAP sensor or Idle valve...sounds like she needs to be scanned