98 S70 RPM's jump up then stall when car is stopped

The first part is magic, (EFI system and good engine)
but?????
gauges go off can be a bad fuse or loose fuse to cluster.
when any Gas engine stalls. the RPM drops to 0
the EFI sees that and goes to key on, not started self test.
this is covered in the operator guide.
key light comes on KEY LIGHT, HUMMMMMMM
Let me look at the waring lamps, there are many.
one light is engine overheat that is logo of a thermometer but looks
just like a key.
http://dashboardsymbols.com/the-symbols/
is this a engine stall post
or engine runs perfect all the time , but only my dash acts funny
you didnt say?

key lamp?
immoblilzer (big trouble there)

a stall is anytime engine RPM drops to zero. called a STALL.
if your cars engine stalls, post that way, the lamps are all normal
on at any stall, on any car.
at a stall , rpm goes to zero and MPH drops to zero as you coast to a stop , stalled. that is all normal for any stall.

if engine stalls.
that is complex.
here is list.

1. lost 12vdc power to EFI system. (battery died, fuse blown)
2. never gave your car the 60,000 mile tune up, see opr, guide.
3. bad fuel.
4. overheated the engine.
5. engine oil gone, never ever checked at each fill up.
6. other more complex EFI system parts failures, ASK.

This could be a lot of things. one of the tests that you need to run is shift it into nuetral when you are stopping and see if it still drops RPM and wants to stall. This will eliminate or conclude that it is a transmission issue. If it is still doing this then it could be a vacuum leak or fuel delivery issue.

I had same thing happened but was battery but you have replaced it thinking your alternator needs replacing now for your battery is getting wrong charge and fluxrating

define haywire.?

dead, off, and is odometer off too?
stopped, you mean the car stalled?

when the engine stalls you see the following (driving school 101)
1: quiet , now. from engine.
2: steering and braking get harder. up to 6 times harder.
3: RPM tachometer shows 0 RPM.
4: Speed drops and lands on zero as car stops or you run out of down hill free right.

so did engine stall? does it restart,?

sounds like a coil issue

Have the car checked to see if it has a speed sensor it could be giving you a bad reading.

Sounds like a fuel filter problem, replace your fuel filter. Call an auto parts supply store for the location of the filter

Have tried getting quality gasoline at a reliable station? It sounds like water in your fuel system. Add a couple bottles of Iso-Heet fuel system dryer everytime you fill up the next few times, and that should cure it.

If the air pump system is setting a fault, then the check valve is probably bad. If it is not addressed, eventually exhaust gases can run back through the air hose to the air filter housing and destroy the MAF sensor, causing these types of problems.

The fuel pump also fails and is a more likely cause of the stalling, and it wont set a code.

These models had a problem with the electronic control modules, and this is what controls the idle speed. Sometimes they just need to be cleaned, but sometimes they are defective. Volvo has extended the warranty on some of them to 10 years or 200,000 miles if it is found to be defective.

http://repairpal.com/volvo-s70-1999