engine problem for 2001 renault laguna 1.8

[grahamf1838]

hi, can someone please tell me what the problem is- every so often the revs randomly go to about 2500 rpm, and the accelerator has no affect. when driving it is ok, provided i don't meet any hills or t/ lights, in which case i usually have to turn the engine off. turning it back on is never easy. a symptom of this is the temp guage dropping to zero. it usually lasts about 15 min at a time and on nearly every journey over the last 2 weeks

please advise...
thanks in advance
grahamf1838