Voyager 3.3 Problems; Stalling and Oil Pressure

[Robby2612]

My parents have a '91 Plymouth Voyager SE 3.3 V6.

They were driving it the other day to take a boxed christmas tree back to a store for a refund on it, but they had some problems with it while they were out.

There were 2 major problems:

1st:
The thing ran fine at first, but when they went onto a major road and accelerated from a stop light, the thing stopped picking up speed when it shifted into 3rd gear at 35 MPH. Basically, at 35 MPH, it hesitated, didn't want to go, and it started to lose speed even though the pedal was floored. It then went to 0 MPH. The engine then stalled and quit on them. He put the shifter in "N" mode and started the motor back up and put it back in OD and tried to go again, but the thing wouldn't hardly go. It sputtered. Instead, they had to drive slowly to a parking lot and park it. He opened to the hood and took a look at it.

2nd:
Here's the 2nd issue...the oil pressure.
My parents reported that oil the pressure gauge was reading virtually "zilch". Basically, they said that the gauge was registering pretty much no oil pressure that day. He mentioned something about the Check Gauges light being on or something too.

Here's my take on this. I borrowed the van a week before this during Christmas week. It seemed to run ok. It didn't seem to skip/hesitate or anything. Oil pressure was ok at least when I drove it; 1/4 at hot idle and a little below 1/2 when going down the road.
My dad told me to check the fluids when I put gas in it, so I did it. The oil checked out fine; it was full, though it looked dark brown/blackish. eh
According to the oil bottles I see in the van, he runs 10w-40 in the engine.
Now, the coolant. The coolant was low. I had to add almost a gallon to bring it back up to level. The burp tank was way down and has almost pure water. The radiator itself? I saw almost no coolant in it. The van never ran hot, though.

We got the van towed from the parking place where we parked at, and it's now at a shop. They are looking at it. We should know something by Mon.

Overall, what could be going on? What do you think should be done and do you think could be causing the above problems?