Gear keeps deploying down, about once every 40 minutes. Several times within a 2 hour flight. Any idea's please?

Are you selecting the 'off' position with the gear lever, or just leaving it in the 'up' position?

Unlike a lot of other jets, the 737 has an off position for the gear lever, and you should be using it because it disconnects the Hydraulic A system from the gear retract mechanism to relieve the hydraulic loads on the system and also engages the uplocks, this is different to a lot of other aeroplanes and is probably not helped by the fact that the 737 doesn't have gear doors on the mains either. With the gear lever in the up position, the gear is merely up against the limit of travel and the rotating uplocks have not swung into place, they do that when the off position is selected. Theoretically, this means the gear could lower itself if there was a hydraulics issue of some kind and the gear lever was not in the off position.

If none of that is the case, it might be something more prosaic, such as your keyboard's G button sticking or a button on a flight controller sending a spurious signal to the sim. I guess you could test that by leaving your computer for an hour with a notepad document open, if you came back to your PC and saw it had typed GGGGGGG for you, you'd have your answer. Keyboards can usually be cleaned, opening them up by undoing about five million posi screws on the underside (some of which might be concealed by rubber feet or stickers) and giving the interior a bit of a spruce up, but it won't always work, they do eventually go U/S and the first sign of that is invariably some keys misbehaving.

Alan, Thank you so much for helping. 

I always have used the 'Off' position after the gear has been stowed. So It can't be that.

Re; your second suggestion I am testing now.

I have even deleted the 'G' in assignments opting to raise the gear manually and that never worked either.

We'll see what happens today and I'll update you with good news, I hope.