Volvo Repair: 2000 S80 Tranmission Slipping, lease deals, filter change

Question
I have a 2000 S80 (Engine B6294s, Transmission GM4T65A), I am really getting worried about the tranmission. When starting from a stop, the transmission sometimes slips.  Very intermittent.  We owe another $6k on the car and it is only worth about that.  It only has 89,000 miles on it, but we are going broke owning it and it is our primary transportation. We can't afford a new transmission. We have already spent our tax return and bonus on it. Nothing left. The warranty ended last year. Everything is going wrong with this car. I put Lubegard in the tranmission when we received the car approximately 50K mi. I was not expecting a trans. prob. this early. Anyway, the code from the engine said it needed a shift solenoid.  Is this serious?
The part is only $27, but when I looked up the instruction on VADIS, it is pretty difficult to replace. Will that fix it? What if we just keep driving it? Will a flush and filter change buy us some time? Any advise is greatly appreciated.

Answer
Brian, Im sorry you spent all this money on these horrible cars.  It is by far Volvo's worst mistake ever.  I have seen techs put that lousy part in cars and it still doesnt help.  That stuff you put in the trans most likely killed it.  It happened to me the other day.  I had bad fluid on a S70, did a drain and fill and within 20 miles, the trans dumped.  You can try a flush service and it might by you time but,  I would trade that thing in on something else.  Volvo has some great new lease deals and Im sure they can get you out of that thing.  Roger