whats wrong with my 2002 mondeo petrol!?

here goes i will try un give as much info as i can. Up until the other day my mondeo was perfect. However driving home the speedo stopped working and the car lost speed and went all jumpy and wouldn't go above 20 miles an hour. The jumping stopped if the cultch was depressed. The engine management light then came on! I pulled over and turned off the car all ready to call the AA, when i restarted it all was back to normal so didn't think anything of it. Just thought the car was having a moment of madness, and up until this morning was running fine when it happened again however today no engine management light on but no speedo and jumping under power and struggling to get up above 20 miles an hour. These is also a sound coming from the front left area under the bonnet when it does this. Hard to explain the sound but like a knocking banging maybe just something loose as the car jumps not sure. Can anyone pls help as i'm really short of money so would like some ideas before i go to the garage so they don't rip me off as it's £40 minimum to read the engine data. My car is a 51 reg mark 3 mondeo ghia 2 litre with only 88k no the clock if that helps.