LAN keeps slowing down

As I see it you either have a congestion on the lan or the database (server) cannot handle the in/out requests.

If it is the database then the slowdown should occur when a lot of requests are made at the same time (so quite a number of users accessing the database at the same time). Furthermore, is the database server dedicated to the database or a mixed server running all kinds of services? Any service which is being used more can slow down the entire server. Is that server also used as print server? The latter could be the reason why both slow down at the same time.

If it is the network then there are a miriad of possible causes and you really need a network technician taking a look at it. You could check the following:

1) Are you using switches or are you running an older network using hubs. Hubs are generally not used anymore but on older networks they are sometimes still used. When a computer is plugged in a switch messages are delivered between sender and receiver. When computers are plugged into a hub all lines are open meaning everytime a computer network card puts a frame on the wire all other computers have to stay silent. If another computer transmits a frame at the same time a collision occurs and all computers halt transmission for a short amount of time. A lot of collisions means a lot of down time and thus slow network response. This usually happens when too many users are on a single collision domain (all users plugged in hubs are on a single collision domain).
2) As you said 'manufacturing' I am assuming you have heavy machinery. If heavy machines (actually even lighting can do it) run near ethernet cables they can cause noise on the line causing bad connections and a constant need for network machines to retransmit the same frames and thus slowing everything down. If every time the network slows down the same machine is used you could check whether or not it is too close to the network cabling.
3) Is there a user who periodically transmits large amounts of data? This can also be caused by a failing / failed network card, when the cards die they can send a lot of trash over the line. You can check the switch where all users are plugged in, if one user is constantly sending data you can see the transmission light burn constantly on that port. A constant burning light indicates constant traffic (after which you need to find out why there is constant traffic). Furthermore, constant traffic can be caused by a virus sending out lots of spam though I do believe a lot of those spam viruses are smart enough not to overload the network so they remain undetected for as long as possible.
--> For example, when I send a complex print job to our office printer it looks to everyone the printer has frozen. The truth is it is just processing my print job and during that processing time the printer does and shows nothing. There was a point where the printer froze for about 15 minutes processing my print job(s), once they even rebooted the printer .
4) There are too many broadcasts traversing the network. Sometimes misconfiguration or other errors can cause a computer to keep on sending broadcast messages. These are transmissions that go to each and every computer even if you use switches and thus every computer has to listen to the broadcast (and after receiving it decide whether to use it or not). When you receive a broadcast the line is in use and you cannot send data.
If you want to get a little technical, you could use a network sniffer to take a look at the traffic being sent over the network at the moments the network gets slow. If it are broadcasts then they should pop up in the network sniffer (try Wireshark). Anyway, in wireshark you can set the networkcard to promiscuous mode so it will record every transmission for you, then when the network congestion occurs you can check wireshark to see whether or not network traffic is higher on your segment at that point in time. But do know, it only measures your network segment. Thus, if you are in a switch you only see broadcasts and messages directed at you. If you are in a hub you can see broadcasts and all messages directed at any user in the collision domain.
5) If your network segment is clear, try checking network traffic on the other segments between your computer and the server. If it is the network there has to be a busy segment in there. You could try checking this at the switch, it might have diagnostic functions.

Note: If the network is large there are so many possible causes that a network technician should take a look at it.

This is all I can think about. Good luck with finding the cause as those network slowdowns can get really frustrating.