Do i have a transmission problem or what

I have a 1999 Astro LS passenger van vin. W with 117,000 miles . Having surging problems that occure between 45 & 55. Runs fine until it warms up. In drive transmission does not seem to hold its gear it feels like engine increases rpm then shifts, slows down does it again and keeps cycling in that manner. wondering if it could be the converter locking then unlocking. Also there is a rattling noise with light to moderate trottle pressure while acellerating in all gears noise is more noticeble on drivers side. When towing small travel trailer problem is still there in 3rd gear. Any information would be appreciated. Roter:confused:

What does the fluid look like? Has the trans been serviced (fluid and filter) lately?

It has been about 20 thousand miles since service, oil was clean and clear with no odor.

Clean fluid without signs of burning is a good indication that the 3-4 (direct) clutch is not slipping. That is a common problem but burned fluid is usually a result. Your description could be of the TCC locking and releasing. If your van has a Tow/Haul mode try engaging that to see if the problem continues. In that mode the TCC is held off longer and line pressure in the trans is regulated higher to provide more clamping force on clutches.

A scan for powertrain error codes might also reveal something.

Thank You for all your help. Decided to have the transmission fixed. It is completly rebuilt they found the pump was badly worn along with the tork converter. It is running just fine now. This car did not have tow mode. Roter