I have a 200) Uplander with the 3.5 engine. It has 135,000 KMs. When cruising at app 100kph, the transmissions torque converter is routinely going in and out of lockup ( alternating between 2000rpm and 2100rpm) It will do this back and forth every few seconds. Sometimes as long as five seconds or so. If you push the van up to 110-115kph The converter seems to stay in lockup and only comes out when climbing a rise. Is this normal. Or a sign that there's a problem?