MX5 stuttering/hesitation issue

[thecremeegg]

Cross posting this from another forum - it's an MX5 mark 1

So car was running fine until this weekend I undid the airbox and moved it aside whilst I changed a hose. Put it all back together and it ran fine - did a 15 mile round trip and it was faultless.
Come to drive it today and if you press the accelerator down it stutters and hesitates quite badly. I drove the 5 miles to work and it was doing it all teh way. I then stopped, dropped the missus off and drove onto work, whereby it ran fine.
Come to the end of the day, problem is back!
It was lessened by the end of my journey home but still there. Happens primarily at low revs, however I sense a tiny hesitation a bit higher up but not sure if that's just me.

Now I'm thinking after googling that things like spark plugs, coil pack etc, however halfway home on the test drive I did put £20 of fuel in.....would dodgy fuel cause this problem?

 

[thecremeegg]

Checking in what way? The intake pipe is sealed well from what I can see/feel?
Sorry, not that good at this stuff!

 

[thecremeegg]

Well I found a split in the crossover pipe thing that might be the cause? Will get a better chance to look at the weekend so will probably try some new leads etc as well.