Windows and Macintosh is what I own in my house. The IMAC that I am using has 10.4.11 the down side is that it has the most malware. Discovered from working in IT for nearly 10 years, that there are 43 different types of malware. As for Windows well last time I checked was last year and there are over 876,000 diferent types of malware. These numbers for both OS's are referring to world wide, after all when you click a link it can be for America, right? Wrong it can be American company, however they can hire some other company located in another country for less money. Part of that less cost is the dollar difference another is security that's setup on their Servers. Now they may have the best, just remember your computer is routing to every sever between where you are and where they are.
Now the reason I said all that above is to help put things into perspective that its extremely unlikely that you have malware issues such as a virus or trojan on your computer. Technically it is possible though and I would not bother with doing any scans.
Recommending that you do like Raven had said. Check with the ISP and before doing that you want to be confident when talking with them. Many people are not familiar with Macintosh OS and they will say it must be a virus. So you must tell them No it is not and no I don't go to sites that have that higher risk such as porn.
Secondly do speed tests see what you are going and right that information down tell them what site you used.
Thirdly the next testing is doing ping must ISP's do not have tech's that are trained in Mac's and you will need to tell them. Yes I know how to find ping yes I did that and here are the number's. Yes I did a trace route and here is that information. Yes we can do that again with you on the phone.
This will earn some respect from their tech's and remember be patient and of course nice. I'm sure you are this way only a friendly reminder. Because working with such issues that you mentioned above can be frustrating even annoying at times to.
Fast and easy way to find PING and Trace Router if you are not familiar with OS X is using Spotlight. Here are the steps ...
1. Upper right corner you will see what looks like a blue dot, look closer it's a magnifying glass.
2. Click on it and type in there "utility" you will see a list of things.
3. Click on Network Utility
4. A window comes up and you will see a serious of Tab's.
5. You are looking for PING tab and Trace Route tab.
6. We will start with PING click it.
7. type in "www.yahoo.com" you will see an example of what to type also on there.
8. Click on the word "ping".
If you get packet loss then you have started to narrow down where the issues are coming from. If there is no packet loss then it can be lag, which also explains the issues you had described.
Typically your milliseconds when doing the trace router should be under 100 the lower the better. You will not reach such numbers as an average of 15ms or 3ms for an average. Don't try to push for that you are just spinning your wheels. If you are over 300ms for an average your ISP has issues they need to resolve. If over 500ms you need to get them to resolve this and its most likely all on their side.
Also when was the last time you Restarted the computer?
How many applications are you running?
How many tabs do you have open in your browser such as Safari / Firefox?
Recommend you use Firefox it works faster on Windows and on Macintosh OS X. If you close Safari when you have many tabs open, well you lose all those tabs. Not in Firefox.
If you are using Safari and it crashes you lose the tabs you had open.
If using Safari then go to a page once in awhile it will give this error "Page Can Not Be Displayed". Then you think its an connection issue or once inawhile a computer issue. It's not the computer it is the application - Safari.
Do NOT use Internet Exlorer (IE) any of the version on Macintosh and try to avoid using IE8 on Windows.
If you have any more questions just ask. Hopefully these suggestions and recommendations have helped you. One last thing this link shows you some short cut commands that you can use on Macintosh. http://support.apple.com/kb/HT1343?viewlocale=en_US