98 Avalon 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 the it
will happen every day for week.

No codes are generated.

Any help would be appreciated