Trans or engine problem??

As the weather has been getting colder my grand cherokee has been getting worse. Its a 99 with a 4.0 4wd laredo. In the morning when the vehicle has sat over night, on the way to work the first few stop lights im at, foot on the brake, the rpms begin bouncing down to about 800 and then come back up, sometimes the trans feels light its trying to make the vehicle buck forward even though ive still got my foot on the brake, other times it just doesnt seem to want to go for a second and then kicks in. The vehicle only seems to do that stuff when its cold, after its warmed up itll still do it at a light, the rpms dropping and coming back up, but the trans doesnt seem to flip out like before it got to temp. Any ideas what to look for or where to start?? theres zero codes so says the snap on scanner, and everything seems to look fine from that aspect

My older vehicles also have been more sluggish in cold weather.

When one of my vehicles had similar problems it was the fuel line either the pump or filter needed replacing, can't remember which.

Check transmsision fluid level when hot engine, I mean let warm up for several (may be30 mins.) then check level and quality( red clolor, viscosity) of fluid ( in PARK).

fuel pump failing sounds more likely to be the problem,,,,,though poor battery connections,,poor battery,,weak alternator will also give engine rpm issues.....

How about your plugs? If you haven't replaced your spark plugs in the last 30000 miles, it's probably time. You said the issue started when it got cold. Any weakness in the electronics will usually show up when the temperature drops.

Also, how many miles do you have? On the '99 sometimes the Crankshaft Position Sensor can cause odd shifts, lurching, and engine anomalies. On my '00, it would shift into different gears, idle oddly, and quit a couple times on me on the highway. If you're over 90k miles then the CPS might be your issue. The only codes I got with mine was "multiple misfire". There was no code for the CPS...at first.


edit: as to the engine/transmission question: does it do it when you're at a stoplight, in neutral? Agree with other poster suggesting to check fluid level....although the 00 suggests (on the trans dipstick) to check with engine hot, idling in neutral.