Cat 950 loses drive when cold

Old Cat 950. Don't know what year, but to give you an idea, it has air over hydraulic drum brakes, full air park brake, twist grip transmission control lever, and steering box type hydraulic steering.

Most of the time it works fine.

But occasionally, when it's cold, you start it up, let it warm up for a little bit, then when you try to drive off, it has no drive in any gear or any direction. After a while it comes good by itself. Then could go for ages before giving trouble again.

Oil level is good and the oil doesn't smell burnt or anything. Park brake works well and releases reliably.

I have pulled out the transmission screen, and apart from a few filings on the magnets it was pretty clean. I was surprised though, that the screen housing was empty. It's been sitting about a week since it was last driven, but I thought it would be full of oil? Is this normal? Seems a big job for the pump to draw oil all the way up through it if it starts off empty each time.

Any other thoughts about what it might be?

Is the oil level being checked correctly, with the machine at normal operating temperature and idling with trans in neutral..?
Air leak somewhere on the transmission pump suction side..?
Look for squashed/hard O-rings, leaky gaskets, slack hose clips, etc, between the transmission sump and the pump suction port.
Hard to be specific without a Serial Number.

EDIT: I laughed when I saw what was written on the bottom of the Parts Book page in the photo...….

A S/N would be more helpful than worrying about the year of the machine.

With a S/N it might be possible to determine the pressure test points so you could know if the problem is no oil pressure to the transmission.

Another of the top of head thought would be a problem in the transmission neutralizer system. Something as simple as a sticking or leaking left brake peddle can cause fits like that.

I "lost" a transmission rebuild job on a machine about the size of 950 years ago when I discovered the left brake peddle was sticking down just enough to actuate the neutralizer valve in the transmission! Guy did pay me for the diagnoses and I really didn't want the job!

Serial number is 73J3158M. It's on the engine. I believe that's how they did it on the old machines. It doesn't work in the Cat online system.

Back in the old days when the engine carried the same Serial Number as the machine.
The online system only starts at 73J03957, so it would appear your machine is from before that time.
It ought to have most of the info though. This is an illustration of the lines on 73J3957. The red circled areas are where I'd be looking first, then more off-the-wall stuff like a hairline crack somewhere in the steel suction line or even a crack in the magnetic screen housing or cover.
It wouldn't surprise me that the screen housing had little or no oil in it considering how high it is above where the normal transmission oil level will be. The slightest leak anywhere in the system will allow oil to drain back to the transmission sump.