Strange hunting at speed

[DB_PUG]

So I've got an issues with my 02 172 cup 1st and 2nd gear are fine pulling off is fine but when in 3rd/4th following other cars it won't hold a steady rev it dips and then jolts forward under power it's fine runs fine and pulls fine but as soon as you are following behind somebody in a queue I slips back into jolting forwards like it's hunting on revs plus the speed needle seems to fluctuate quite a lot when this happens , I've just had the lambs sensor swapped due to a decat pipe being fitted but it was present before is it worth changing plugs leads and coilpack?

Any help would be great

 

[Vaughan]

Speedo sensor on the gearbox.

 

[DB_PUG]

Would that cause the revs to be unsteady tho?

 

[Vaughan]

Absolutely. My 172 had exactly the same problem -- wobbly speedo needle, a speed surge while cruising. I changed everything like plugs, crank sensor but it made no difference. New speedo sensor fixed it.

Non-OEM items are pretty cheap.