With the engine mount's you look for small crack's or play. To find out if there is any play you can try and look for a clear gap in the mount's, or get someone else to rev the engine gently and look for any obvious wobble that may look a little extreme. Also one favourite is crap petrol. I was using Tesco Momentum 99octane until I drained some and saw the crap that was in it. Was giving mine a little cough but affecting performance big time. I know it's dearer but I changed to Shell V-power. Cough disappeared after 2 tank's full. Will never use any other now. You could also use a pry bar or Crowbar to rock the engine mounts to see if there is any play. Without seeing it the only other thing could me a blockage in the fuel inlet somewhere. That one won't be easy to find tough. My advice is check the mount's, change the fuel and give it a good long run to see if you can blow anything through. Let me know what happen's mate and I will try to help. Good luck!