Sounds like a general maintenance issue, maybe... one of your coils is going, maybe.
Is the check engine/service engine soon light on?
Do you have a scanner or code reader - you'll need a very good one because if the SES light is not on you'll be hunting for a pending code.
Here is the insight:
The rear plugs/wires on the Riv are hard to get to. Some (lazy) mechanics will leave them be but say they serviced them. The OE plug terminals (in the boots) click on with a distinct snap. They can be pressed on so they are close, but will back off after a while. Then you get hi voltage arcing either at plug or coil depending on conditions. It may take a very long time for the wires to back off enough to cause the issue. When it finally happens, all that hi voltage with nowhere to go will wreak havoc, and the engine will still run somewhat.
Another thing that happens is the wires just wear out. I'd expect less than 100,000 miles from Delco wires and they are the *good* ones.
Another thing that happens is the plugs wear. OE iridium plugs will get you about 100,000 miles, platinums about half that, standard ones about half again.
Why all this crap about ignition and plugs? Your vehicle is moving at a constant speed; I have a hunch that arcing may be radiating into the wiring (at the ignition module, which sits under the coils) and upsetting the vehicles' speed signal to the ECM/PCM and thus to the speedometer head. THIS IS JUST A HUNCH.
meter to check wires.
This isn't a typical failure for the Riv per se but is typical of under-maintained electronic ignition systems. If you do the maintenance yourself it won't be that bad to set it right and you likely can do it with a mix of junkyard and OE parts.
What I suggest you do first is check all the plugs and wires. A loose wire (and not much else) will kill a coil, check the coils too. Wires die with age and use, so you are also looking for a bad wire. Bad means open, intermittent or otherwise compromised. If you need instructions on checking coils do a search on this site, there are posts and I think a write up about that.
Another issue could be a bad alternator but I would think you would notice surging lights, radio, and other foolishness not just the spiky speedo.