Are you still having issues with the gauge failing or is it just slow to warm up and running cool while cruising?

Your symptoms of slow to warm up and running cool while cruising at speed are classic signs that your thermostat is bad (sticking open) - as others have already pointed out.

Cheers

It is slow to warm up and runs cool while cruising but the gauge also occasionally fails when the ambient temperature is low. Usually I can avoid the gauge failure if I let it warm up at idle on cold days.

Sorry to revive an old thread, but it doesn't sound like the OP has found a solution and I have encountered the same issue.

My 996tt tip has 113k miles. I recently went through the dreaded massive coolant dump. I opted to have every coolant fitting pinned among a plethora of other preventative maintenance. On the drive home from the shop (1 hour drive on the interstate) I noticed that the car was not coming up to temp. It was hovering at the mark between 100 and 180. I am told this is approximately 120°F - 130°F. A few days later I took the car back to the shop to have the thermostat replaced. About 20 minutes into my drive home the temperature dropped again.

Not sure if it is related or not, but I also incurred a P0491 and P0492 CEL after the overhaul. The CEL comes on only if I let the car warm up at idle for an extended period of time.

I am taking the car back to the shop for further diagnosis. I will bring up this thread and the suggestions listed. Crossing my fingers for a speedy resolution. I'll report back soon.

All fixed
 

Got my car back yesterday. The root cause of the CEL was a loose vacuum hose leading to the secondary air pump. The root cause of low temperature ready was a faulty temp sensor.

Moral of my story this far is, if you overheat the car and/or have a massive coolant blow out, change the temp sensor and thermostat while everything is apart.