The problem may not be in the charging system, but the clusster(gauge), GM has had a issue with the clusters in the 2003-2005 trucks. The cluster will be replaced for free for your truck since it has less then 70k miles. I would take it to your local dealer and tell them that the voltage gauge sticks at times. They should order one for you, takes 1-2 days and replace it for you at no charge. If this does not fix the problem, you will have a new cluster at no charge.Unless the battery light comes on, the alternator should be good. This is a copy of the GM bulletin.

THIS BULLETIN IS BEING REVISED TO INCLUDE CERTAIN 2005 MODEL YEAR VEHICLES IN THE U.S. ADDITIONALLY, FOR U.S. 2003-2005 MODEL YEAR VEHICLES, GM WILL PROVIDE THE INSTRUMENT PANEL CLUSTER (IPC) ONLY, AT NO CHARGE TO THE CUSTOMER FOR VERIFIED FAILURES BETWEEN 70,001 MILES (110,001 KM) AND 80,000 MILES (130,000 KM). ANY ADDITIONAL COSTS, INCLUDING LABOR TO REPLACE THE INSTRUMENT PANEL CLUSTER, WILL BE THE RESPONSIBILITY OF THE CUSTOMER. DEALERS WILL BE RESPONSIBLE FOR THE RETURN OF THE CUSTOMER'S IPC BACK TO THE ELECTRONIC SERVICE CENTER. PLEASE DISCARD ALL COPIES OF BULLETIN 07187B, ISSUED SEPTEMBER 2008..

CUSTOMERS ARE BEING INSTRUCTED TO CONTACT THE DEALERSHIP TO ARRANGE AN APPOINTMENT IF THEY BELIEVE THEIR VEHICLE HAS THIS CONDITION. THE CUSTOMER IS BEING ASKED TO PROVIDE THE VIN SO THE IPC CAN BE ORDERED IN ADVANCE OF THE SCHEDULED APPOINTMENT. THIS WILL ELIMINATE THE NEED TO KEEP THE VEHICLE OVERNIGHT. DEALERS ARE TO OBTAIN THE VEHICLE MILEAGE WHEN SCHEDULING A SERVICE APPOINTMENT. THE VEHICLE MILEAGE WILL BE REQUIRED WHEN PLACING AN ORDER FOR THE IPC.

Some customers of all 2003-2004 model year and certain U.S. 2005 model year Cadillac Escalade, Escalade ESV, and Escalade EXT; Chevrolet Avalanche, Silverado, Suburban, and Tahoe; and GMC Sierra, Yukon, and Yukon XL vehicles have reported that one or more of the instrument panel (IP) cluster gauges stick, flutter, or become inoperative. This may cause inaccurate readings, including the speedometer and fuel gauge.

This special coverage covers the condition described above for a period of 7 years or 70,000 miles (110,000 km), whichever occurs first, from the date the vehicle was originally placed in service, regardless of ownership.

Dealers are to replace the instrument panel cluster after the condition has been verified. The repairs will be made at no charge to the customer.

For verified failures between 70,001 miles (110,001 km) and 80,000 miles (130,000 km) on U.S. vehicles, dealers are to provide the customer with an instrument panel cluster (IPC) only. Any additional costs, including labor to replace the instrument panel cluster, will be the responsibility of the customer. Dealers are responsible for the return of the customer's IPC back to the Electronic Service Center.

Involved are all 2003 and 2004 model year and certain U.S. 2005 Cadillac Escalade, Escalade ESV, and Escalade EXT; Chevrolet Avalanche, Silverado, Suburban, and Tahoe; and GMC Sierra, Yukon, and Yukon XL vehicles built within the following VIN breakpoints: