Jimny losing power over 50mph

Just a recent development on my 2002 jimny. It drives fine at lower speeds but once I'm on the motorway and try to go above 50-60 there's just no power at all. It even starts to slow down on small inclines. It revs fine in neutral and is definitely firing on all cylinders. Any thoughts folks?

One thing to note. I hadn't driven it in a few weeks recently and when I tried to start it, it took awhile to actually tick over and I had a huge belch of grey smoke once it started. Been fine since though.

They like to rev.... below 3500rpm, an incline is enough to slow them down..... peak power is around 6000rpm and peak torque at 4100rpm.

It feels a little harsh to thrapse them but the little engines lap it up......

sniper

Well aware of how they perform, I have it 2 years which is why I'm posting here as something is definitely not right with it. Previous to this it used to reach 70mph no problem at all, but now it struggles to get to 60 and slows down in the slightest of inclines whereby more throttle doesn't do anything.

Blocked exhaust?
Rust develops inside the exhaust if left standing for any length of time, starting shakes it all loose and it bungs up the first true restriction, usually the back box.
Just the first thing that entered my mind upon reading your problem. I guess if you cant find any other reasons first (last resort) you could try running it without the back box.

Sounds like Mike said, the exhaust could be blocked. The huff of grey could be the catalytic converter breaking up.

Ah yes forgot about the manifold CAT, as Lambert said this can block the middle CAT, moving the blockage a bit further forward.
Just another simpler possibility, your brakes are free aren't they?
May be worth a check to see if you can push the Jimny, handbrake off.
Mike