The pedal is sticking down, and you cannot get gears sometimes...
The pedal can stay down if there is air in the system or the master is shot etc... I have also seen this when the clutch cover diaphragm spring packs in, but the randomness of your issues does point to the hydraulic system.
Is there any fluid leaking from the master, or from the slave?
The reason you cannot get any gears could be because your clutch is not disengaging because there is no fluid or air in the system so your hydraulics are down the pan. The system on these is notorious to give strange symptoms like this when failing or even after install of new parts before proper bleeding.
Next time it won't let you select a gear, switch the engine off, put it into 1st, take the handbrake off make sure there is nothing in front of you, push the clutch to the metal and switch on the engine, if the vehicle moves forward then the system is not disengaging, or if the vehicle feels like it is trying to move forward take this as big time clutch drag through lack of hydraulic pressure or worn seals letting fluid leak back etc.
The reason I think you have this also stems from you saying on one occasion you could get forward gears but no reverse... syncros on forward gears but not reverse.
If you do diagnose it as hydraulics best bet is a good quality rebuild kit for both slave and master unless you know one to be in good order. Or a new good quality slave and master. I've done parts of hydraulic systems in the past and it always seems like a month or two down the line you are doing the other end and it's the bleeding bother all over again.