Toyota Camry 1999: What is causing it to lose power?

MY 1999 Toyota Camry Conquest sometimes loses power and starts kangaroo-hopping as if it's starved of fuel or misfiring. When I turn it off and rest it for a while it drives OK. Some days it is hard to start and I have to keep the revs up to stop it from stalling. It has done 112,000km and I've just had it serviced. New petrol and air filters and a new timing belt were fitted, and the injectors were cleaned.