Have you tried changing the motor brushes? These symptoms often indicate that the brushes are worn.

Well it is ok. Once everybody was a beginner. Started his hobby with learning by doing. But please, first of all tell us the article number of your start-up set. Only with this number it is possible to identify your loco and find it in Maerklin's databse. For these sets it is so far not so easy, because the locos do not have an own single article number. They are combined with the complete set. But you can find this loco under another number as another model at the database. So please tell us the set number.

Also normally (exceptions possible!) in the attached manual is at the end an explosion drawing of the loco plus a list with all spare parts and their numbers. So you can identify the spare parts number for the postion you need to change.

BTW
Happy new year - happy new hobby - happy welcome at MUN

As Ray says new brushes might help but also make sure the loco is clean by cleaning wheels and pickup. Also clean off any old oil and use new oil sparingly.

Marklin have oil for their locos. However it seems odd that the loco goes fine in reverse so cleaning is only a first step as other treatment may be required. Your dealer should help with what you need or you can look at the Marklin website for parts etc.
Do you have another loco that runs OK.
Members on here are great with advice but it can be difficult without actually seeing the loco to ascertain what is wrong.

I tried to find a spare parts list for your loco, but I was not yet successfull. Maerklin has and does offer plenty of these V 100 ( 212, etc.), but you need inforamtion about the built in motor for the brushes. Your set is from the year 2004/2005, but should have already a HLA engine inside. Perhaps you open the loco and get a look inside to evaluate by yourself, which motor is inside or set up a picture of the engine here at marklin-users.net.

Probably you do not understand the German language, nevertheless I set up here a few links from a Swiss guy's excellent homepage. But only the section about motors giving you as a newbie an idea what we talking about

https://www.stayathome.ch/motoren.htm

motor conversions (because of the detailed pictures and the picture of the cole brushes at the end of the spare parts listing for Maerklin)

https://www.stayathome.ch/motorumbau.htm

and motor service.

https://stayathome.ch/Wartung.htm

Hope it helps you changing your cole brushes.

Finally a very old service manual from the analogue era of Maerklin, but it is in English and many description are still valid and helpful today.



E.g. inside the brochure you see an explosion drawing and a spare parts list. So you get informed finding the correct article number for needed spare parts.

... and a translating software

https://www.deepl.com/translator

The spare parts should be the same as for set no. 29755

The spare parts number for the brushes is 601 460

https://www.maerklin.de/...e/details/article/29755/

Per.