Engine surging, intermittant stalling Volvo 240? Turbo

[LynnR]

4 speed manual transmission - over 300,000 kilometers on the odometer- former owner said -rebuilt engine with 80,000 kilos on it - PW , PDL, manual crank sunroof, very clean interior, few minor fixes needed on some of the interior.
I purchased this car for $800.00 -replaced all tires twice (1st all season radials then to ice tires for winter mtn driving ($1200.00) new clutch-$300.00 new spark plug wires $100.00-was supposed to fix the surging-mechanic said he put the engine on a scope and thought surge was due to cracked spark plug wire., new passenger side floor-$600.00 ,replaced the headlight switch $35.00( did this job myself) Whoever owned this car before did some of their own wiring. I can for instance put my power windows up and down without the key or ignition being on. The power door lock only locks ,will not unlock ,etc but I do love this car. I have been experiencing intermittent surging while the car is idling ,and some seemingly random stalling. it does not seem to matter if the car is warm or cold, although it seems to happen more in the damp weather. My oil pressure is jumping around when this happens , staying around the 4- 6 range . The voltage meter today was hovering in the very low range but staying mostly out of the red. Could this be a sign of impending alt or generator problem? A short somewhere? This car starts like a champ,no problem there . I am mostly highway driving and this is my only transportation to get to work,etc. so I need this car to stay safe and reliable. Any ideas on what could be causing this problem? A look under the hood did not reveal any obvious wiring problems, but I am not even a backyard mechanic. Is this enough information?