1MZFE surging rpm.

98 1MZFE engine has a intermittent problem with surging rpm, it only happens after driving for at least 20 mins. At idle it sounds like it will stall but never does, when driving you can feel the surging and see the

rpm going up and down a few hundred rpm.

If you stop and wait for a while (15-25 mins) it goes away but returns in about 10 mins. It may happen every day and sometimes not for over a week and then it will happen every day for week.

No codes are generated.

Any help would be appreciated.