I have experienced a very similar problem. Best you head over to the Cultists:
Just do a search for 'slide stuck' or similar, and you'll see you're not alone. 150 rounds is not that uncommon for this kind of problem to materialize, though it is rather often.
First, check if you're not accidentally hitting the disassembly button while the gun cycles, that might just get your slide stuck. Although, I do not think I've heard of this happening with a P7M8, as this was mostly an issue with older P7s with raised disassembly button. Since you say the slide doesn't move smoothly even before it gets stuck, that means that is has to be an issue with the gas piston/cylinder.
Cleaning is the word. Make sure you don't use LNR or copper-coated ammunition in a P7M8. Then, check with different ammo, see if there's any variation in when the problem occurs. If it's an older gun, you could check the dimensions of the gas cylinder and gas piston, and inspect the gas cylinder for any visible wear. I've heard, and seen, that over time a groove can and will form opposite the gas port This might just lead to the gas piston getting stuck in this area, though I haven't heard of this happen. Wear on the piston is usually easy to identify, if there's carbon residue, preferably baked on, further aft than the first two piston rings, it's perhaps time to change the piston. And, of course, check if the gas port is free, I use a bent paper clip for that purpose.
Then there's the question of how do you clean the cylinder? As it sounds like there's a fair amount of buildup somewhere in there, it might be a good idea to let it soak in solvent for a while, and then use the piston scraper that comes with the gun to clean it out. After that, I'd use a bronze brush fitting the piston (think it was .270 off the top of my head) and scrub it. It is important to check if there isn't any builup at the bottom (or back, whatever) of the piston, which can be hard to see. Repeat until it works nicely again. Finally wipe it clean with q-tips. Again, make sure the gas port is not blocked. As for oiling the cylinder/piston, I always shoot mine dry.
That's about all I can think of. Heh, sorry for the lenghty post, but it breaks my heart to see a P7 sitting idle.