My fiesta's done 108,000 miles and has been a very reliable car - recently its been having some starting issues in that it will sometimes happily start as normal - but other times will fail to start completely.

Seems to randomly decide if its going to or not. When it doesn't start no amount of cranking or re-trying will help, but if it is push started even in 1st at about 5mph the engine roars into life the second i release the clutch! This used to happen maybe 1 in 10 times I started the car but its more like every other time now. When the engine has been recently run (less than 1 hour ago) it also happens albeit less frequently.

After start up when driving for the first 5 minutes or so generally until the engine has reached operating temp, when coming up to lights etc on clutch depressing engine has a tendency to stall or drop to near stall revs (below 500) and recover. Once fully warmed everything works fine - car seems to run perfectly.

As far as i'm aware the car has had a very mild form of the non starting problem for a long time - my father owned car before me (he did the first 95K miles) an said it would v. occasionally (less than 5 or 6 times a year) fail to start when frosty outside, but he'd leave it for a few mins and would work fine.

I replaced the sparks about a month an a half ago - this 'seemed' to stop the problem but it reappeared again after two weeks, replaced the HT leads last week - made no difference. One occasion where it failed to start I sprayed WD40 on all ignition connections and then it started, but have had no success with this method since.

I'm stumped as to what could be the cause - was thinking maybe replace battery but if it starts first time when its been sat around for a day or two and then sometimes wont after an hour it cant be the charge level, so was thinking maybe some sensors somewhere are to blame? Oxygen sensor old and battered??