1998 GMC Envoy

I WAS DRIVING ALONG AT HIGHWAY SPEEDS WHEN MY 1998 GMC ENVOY WITH 43162 MILES STOPPED ACCELERATING. WHEN I STEPPED HARDER ON THE GAS, THE ENGINE WOULD NOT REV AND POWER BEGAN TO RUN OUT FROM THE ENGINE. THE MOTOR STALLED AND YET THE ELECTRICITY WAS STILL ON IN THE CAR. I BEGAN TO LOSE BRAKING POWER AND STEERING BECAME HARDER. I WAS ABLE TO PULL OFF THE ROAD, TURN OFF AND TURN THE CAR BACK ON AGAIN. BUT THE CAR STALLED AGAIN AND AGAIN, FIVE (5) TIMES IN SIX (6) MILES. THIS HAPPENED SEVERAL YEARS BACK, AND HAS CONTINUED TO HAPPEN INTERMITTENLY. I ALWAYS THOUGHT IT HAPPENED WHEN I GOT BELOW 1/4 OF A TANK OF GAS, BUT TONIGHT I WAS AT HALF A TANK. THE ONLY THING I NOTICED WAS THE OOIL PRESSURE GAGUE WAS AT SLIGHTLY RIGHT OF MIDDLE, LIKE AROUND 60 I THINK, WHICH HAPPENS ONLY OCCASIONALLY. FINALLY I GOT HOME AND, WITH THE CAR IN PARK, PUT CONTINUOUS EVEN PRESSURE ON THE ACCELERATOR BUT THE NUMBER OF RPMS WAS VARYING, BY UP TO 1000 OR 1500, FROM 3500 DOWN TO 2000 -- UP AND BACK CONTINUOUSLY BUT NOT ON AN EVEN CYCLE. IT'S VERY DANGEROUS WHEN THIS HAPPENS UNEXPECTEDLY AT HIGHWAY SPEEDS AND I HAVE BROUGHT IT TO THE DEALER BEFORE TO REPORT THIS (SINCE AROUND 1998 OR 1999) BUT THIS KEEPS HAPPENING. I ONLY HAD THOSE FEW SECONDS WHEN THE MOTOR WAS NOT ACCELERATING BUT THE CAR HAD NOT YET STALLED TO REACT -- PUT ON THE HAZARD LIGHTS, PULL OFF THE ROAD, COME TO A STOP -- AND LUCKILY I KNEW WHAT TO DO OR ELSE I COULD HAVE BEEN KILLED BY SOMEONE REAR-ENDING MY CAR. PLEASE HELP, I AM TERRIFIED OF THE CAR AND HAVE LOST ALL CONFIDENCE IN IT, YET AGAIN.*AK