Rise Clamp Feed

Hi,Before normal running temp engine runs faultlessly. However, once the temp is normal, pressing on accel pedel causes rpm's to increase for moment of a sec before the revs die off (above 2000rpm not at idle), again for a moment of a second, revs then rise again before falling back off and so on. It is possible sometimes to get past this point but then it just happens again above 4000rpm. Once the engine has cooled again there appears to be no problem, until the temps rise again. Using jumper wire method in diagnostic box has no effect on symptom and no fault codes are logged. If I connect a inductive timing gun to the underbonnet battery feed and the other wire to a ground point and the clamp over one of the plug leads (or even put it near the leads) then the problem goes away as soon as you press the timing trigger. Release the gun trigger and the problem comes back. I changed leads and plugs for new ones today but hasn't changed the problem at all!
please help undriveable. John