It's a 1999 Mazda Familia.
When I start driving and the engine's cold the car often stutters and fluctuates between 500 and 750 revs. Then when I accelerate the revs go up but the car goes very slowly for a few seconds, then produces a loud thunk and launches fine (real bloody hassle at a busy intersection). Because the revs were increasing as normal I thought it might have been a transmission issue but the car still stutters and shakes like it's about to stall even when it's in neutral. Also when it's driving and it dips below about 1500 revs, the accelerator will skip for a few seconds before righting itself. This usually happens when the engine is cold and about 90% of the time it goes away after a few minutes when it's warmed up.
I've booked it in for a service this weekend and I'm thinking it might be a spark plug or fuel line issue but I wanted some ideas from someone who knows more than me about cars (~99.9% of all people)
Update: The car now always shudders, so it's getting worse. I've booked it in for a mechanic on Monday.
Update: I've taken it in to a mechanic and he says it's probably the idle air control valve but the bigger issue is one of the coils is buggered so the car's running on three cylinders. It should hopefully be sorted tomorrow.