Nissan Navara power loss

Sometimes the engine starts first time and others the accelerator has to be pumped to get it to start with subsequent cloud of smoke from rear. Revs will not go above 2500 until pumped and then rise normally. Power can be lost at low revs and the engine stalls. The power loss can happen at any time with the engine cold or hot. I have read elsewhere it could be a blocked fuel filter at best and vacuum loss and electrical problems. Heavy Duty Battery fitted about 6 months ago as initially thought that was part of the problem.