I've been looking at jetboils but have heard they have some trouble at altitude.Someone help me out.l need a personal system for cooking food/melting snow while mountaineering.

l recommend a MSR Whisperlite or Whisperlite International. The MSR stoves are the industry standard for mountaineering.

Let us know what you get!

The main thing that u need to look for is, can u use liquid fuel?

Gas canisters lose pressure as u take them up to altitude, which leads to slow heating of ur meal/snow/whatever, so u really need to have a liquid fueled stove.
Of course, a stove that can use liquid whilst out on the mountains, but gas whilst back at camp, is perfect, as gas canisters r very easy to come by, whilst petrol can be a bit harder, depends where u r l guess. You can also usually paraffin, white gas, aviation fuel (kerosene), & diesel. Some might work on meths or coleman liquid fuel, but not usually recommended.

Anyway, good systems to look for r
MSR (Mountain Safety Research, if u wondered) Whisperlite International, which is purely liquid fuelled, & comes at about £75,
Primus Omnifuel or Primus EtaPower MultiFuel, both of which can use liquid or gas canisters & both r £125 (the omni is a small lightweight one, whereas the etapower comes with all sorts of pots & pans, so the omnifuel is more what you'd be looking for),
Optimus Nova+ which is again purely liquid fuelled, & more lightweight but more expensive at £130.
Or if you're just browsing, good features to look for r packsize, weight, how wide & hence stable the legs r & whether or not it pre-heats the fuel by running the pipe through the flame before it curls back around to vent the fuel.
Aluminium windshields & ground shields r good to increase the efficiency.

Hope this helps, but the main thing to remember is, u need liquid fuel at altitude (i.e. not a jetboil, which is also very narrow & tall & hence not too stable)

l have a MSR Whisperlite International which can run on several liquid fuels, l have used it at 12,600 ft (highest camping I've ever done) and it worked almost as well as it does at low elevations.

Of course a heat exchanger and wind shield will also be a critical feature as these increase efficiency and you wo not have to carry as much fuel.

Good Luck