Well, the ignition switch arrived Saturday and I had it in in less than 30 minutes... the black part was separating from the silver part on the old switch and the spring inside was worn. Key has much more tension starting the car now, but the temp gauge still drops out while driving. I cleaned the contacts in front of the battery with all the little black wires on it.....not the problem either. So I'm thinking the harness to the VDO is loose. Good project for next weekend. I have an old one to swap out to check. If this doesn't work, I'm out of DIY checks unless someone has another idea... the strange thing is it always works after sitting for about 8 hours....it works for about 20 minutes or so and then just drops to the bottom of the display like there is no power. it won't work for the rest of the day, and after sitting for about 8 hours it works again, sometimes for a couple days....strange.

Ditto on coolant temp sensor. Check the wiring and the plug. With the hot engine off and the and the gauge at the bottom, use a volt meter and check the ECT sensor for resistance. The volt meter should read about 150 when the coolant is hot. I suspect the wiring.

Are there any fault codes?
--
My back feels better when I sit in a Volvo seat

thanks Greg and Klause......background is last fall/winter the problem started. There was/is a code for the ECT not reading so I checked the plug which was corroded and fell apart when I pulled it apart. I opened the plug and removed the female bullet connectors and spiced in new ones and snapped the plug back shut. The ECT is a newer fae. anyway, this weekend I reworked the wires and soldered in new copper 18 ga. stranded wire. I put a zip tie around the plug to keep everything tight. The solder points are in shrink tubing and then I electrical taped over that just to make double sure nothing was shorting. They say polarity doesn't matter, but I found that the yellow/gray goes to position 1 and brown gray to 2. Position 1 is by the angle part of the plug. Went to the doctor and back this morning (about 40 miles) and no problems. Another thing Im wondering about is when my brother jumped me last winter I reversed the jumper just for a second....this can be really bad....maybe I did something to the new sensor. When the needle drops it just goes without warning and stays as if it has no power. Dont want to drive like this much longer in the summer.....I cleared codes again...will check this weekend. I asked FCP to list battery terminal bolts and nuts.....mine got stripped from all the screwing around this weekend. They said they would have on the site by this afternoon under battery....about 5 bucks for the bolts and 3 bucks for the nuts...just FYI

When your temp needle drops down, if it still does, can you check the codes on A-7, the instrument cluster? There is a code for no signal from ECT sensor. Same for rpm which comes from the CPS.

Jumping backward from another car can destroy the ignition module and/or the engine computer. Hopefully, that didn't happen.

I doubt that the instrument cluster is at fault.

Did you check the ground wires???
--
My back feels better when I sit in a Volvo seat

Needle dropped going to work tonight- will check a7 and ohms hot in the morning thx

Have you tried putting the sensor in hot water with a cooking thermometer and measuring the resistance at increasingly higher water temps? You may find the resistance goes infinite at some point like up around 190 deg (faulty sensor).
--
Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (fixed the brake fluid leak - now on to the ignition)

I don't see your prvious post listing what you checked, so... It sounds like a heating problem. What ever the problem is shows up when the car is warmed up and goes away once it cools down. With a long cool down time, I would suspect something big, like the engine.

How does your coolant temp sensor act as it warms up and once the gauge drops out ?

Greg